[REEF-2062] Migrate from FindBugs to SpotBugs

FindBugs is not supported anymore, so we're migrating to the latest version of SpotBugs.

Summary of changes:
* Update Maven pom.xml files to reference to the latest verison of SpotBugs
* Rename teh related config files and intermediate files and make sure the SpotBugs report is correct
* No changes in code were required.

JIRA: [REEF-2062](https://issues.apache.org/jira/projects/REEF/issues/REEF-2062)
diff --git a/lang/java/reef-bridge-proto-java/pom.xml b/lang/java/reef-bridge-proto-java/pom.xml
index 8fec306..56c17b6 100644
--- a/lang/java/reef-bridge-proto-java/pom.xml
+++ b/lang/java/reef-bridge-proto-java/pom.xml
@@ -283,8 +283,8 @@
                 <artifactId>maven-compiler-plugin</artifactId>
             </plugin>
             <plugin>
-                <groupId>org.codehaus.mojo</groupId>
-                <artifactId>findbugs-maven-plugin</artifactId>
+                <groupId>com.github.spotbugs</groupId>
+                <artifactId>spotbugs-maven-plugin</artifactId>
                 <configuration>
                     <skip>true</skip>
                 </configuration>
diff --git a/lang/java/reef-common/src/main/resources/findbugs-exclude.xml b/lang/java/reef-common/src/main/resources/spotbugs-exclude.xml
similarity index 100%
rename from lang/java/reef-common/src/main/resources/findbugs-exclude.xml
rename to lang/java/reef-common/src/main/resources/spotbugs-exclude.xml
diff --git a/lang/scala/reef-examples-scala/pom.xml b/lang/scala/reef-examples-scala/pom.xml
index 85e0258..57a1dbb 100644
--- a/lang/scala/reef-examples-scala/pom.xml
+++ b/lang/scala/reef-examples-scala/pom.xml
@@ -33,8 +33,8 @@
 
     <properties>
         <rootPath>${basedir}/../../..</rootPath>
-        <!-- Findbugs does not support Scala code -->
-        <findbugs.skip>true</findbugs.skip>
+        <!-- SpotBugs does not support Scala code -->
+        <spotbugs.skip>true</spotbugs.skip>
     </properties>
 
     <dependencies>
diff --git a/pom.xml b/pom.xml
index 8316903..17e9151 100644
--- a/pom.xml
+++ b/pom.xml
@@ -62,7 +62,7 @@
         <maven-checkstyle-plugin.version>3.1.0</maven-checkstyle-plugin.version>
         <sevntu.checkstyle.plugin.version>1.35.0</sevntu.checkstyle.plugin.version>
         <checkstyle.version>8.29</checkstyle.version>
-        <findbugs.version>3.0.4</findbugs.version>
+        <spotbugs.version>3.1.12.2</spotbugs.version>
         <reflections.version>0.9.9-RC1</reflections.version>
         <jsr305.version>3.0.1</jsr305.version>
         <kryo.version>3.0.3</kryo.version>
@@ -361,16 +361,17 @@
                         <includeTestSourceDirectory>true</includeTestSourceDirectory>
                     </configuration>
                 </plugin>
+
                 <plugin>
-                    <groupId>org.codehaus.mojo</groupId>
-                    <artifactId>findbugs-maven-plugin</artifactId>
-                    <version>${findbugs.version}</version>
+                    <groupId>com.github.spotbugs</groupId>
+                    <artifactId>spotbugs-maven-plugin</artifactId>
+                    <version>${spotbugs.version}</version>
                     <configuration>
                         <effort>Max</effort>
                         <threshold>Default</threshold>
                         <failOnError>true</failOnError>
-                        <findbugsXmlOutputDirectory>${project.build.directory}/findbugs</findbugsXmlOutputDirectory>
-                        <excludeFilterFile>${rootPath}/lang/java/reef-common/src/main/resources/findbugs-exclude.xml</excludeFilterFile>
+                        <spotbugsXmlOutputDirectory>${project.build.directory}/spotbugs</spotbugsXmlOutputDirectory>
+                        <excludeFilterFile>${rootPath}/lang/java/reef-common/src/main/resources/spotbugs-exclude.xml</excludeFilterFile>
                     </configuration>
                 </plugin>
                 <plugin>
@@ -379,19 +380,19 @@
                     <version>1.0</version>
                     <dependencies>
                         <dependency>
-                            <groupId>org.codehaus.mojo</groupId>
-                            <artifactId>findbugs-maven-plugin</artifactId>
-                            <version>${findbugs.version}</version>
+                            <groupId>com.github.spotbugs</groupId>
+                            <artifactId>spotbugs-maven-plugin</artifactId>
+                            <version>${spotbugs.version}</version>
                         </dependency>
                     </dependencies>
                     <configuration>
                         <transformationSets>
                             <transformationSet>
-                                <dir>${project.build.directory}/findbugs</dir>
+                                <dir>${project.build.directory}/spotbugs</dir>
                                 <includes>
-                                    <include>findbugsXml.xml</include>
+                                    <include>spotbugsXml.xml</include>
                                 </includes>
-                                <outputDir>${project.build.directory}/findbugs</outputDir>
+                                <outputDir>${project.build.directory}/spotbugs</outputDir>
                                 <stylesheet>default.xsl</stylesheet>
                                 <fileMappers>
                                     <!-- Configures the file extension of the output files. -->
@@ -903,8 +904,8 @@
                         </executions>
                     </plugin>
                     <plugin>
-                        <groupId>org.codehaus.mojo</groupId>
-                        <artifactId>findbugs-maven-plugin</artifactId>
+                        <groupId>com.github.spotbugs</groupId>
+                        <artifactId>spotbugs-maven-plugin</artifactId>
                         <executions>
                             <execution>
                                 <id>analyze-compile</id>