Bump maven-checkstyle-plugin from 2.6 to 3.1.1

Bumps [maven-checkstyle-plugin](https://github.com/apache/maven-checkstyle-plugin) from 2.6 to 3.1.1.
- [Release notes](https://github.com/apache/maven-checkstyle-plugin/releases)
- [Commits](https://github.com/apache/maven-checkstyle-plugin/compare/maven-checkstyle-plugin-2.6...maven-checkstyle-plugin-3.1.1)

Signed-off-by: dependabot[bot] <support@github.com>
diff --git a/pom.xml b/pom.xml
index 2143970..5a9840e 100644
--- a/pom.xml
+++ b/pom.xml
@@ -238,7 +238,7 @@
       </plugin>
       <plugin>
         <artifactId>maven-checkstyle-plugin</artifactId>
-        <version>2.6</version>
+        <version>3.1.1</version>
         <configuration>
           <configLocation>org/apache/commons/functor/checkstyle.xml</configLocation>
           <headerLocation>org/apache/commons/functor/license-header.txt</headerLocation>