Update release checksum algorithms to SHA-256, SHA-512

Patch by Michael Shuler; reviewed by Brandon Williams for CASSANDRA-14970
diff --git a/CHANGES.txt b/CHANGES.txt
index aeb3009..7b88686 100644
--- a/CHANGES.txt
+++ b/CHANGES.txt
@@ -1,4 +1,5 @@
 2.1.21
+ * Update release checksum algorithms to SHA-256, SHA-512 (CASSANDRA-14970)
  * Check checksum before decompressing data (CASSANDRA-14284)
  * CVE-2017-5929 Security vulnerability in Logback warning in NEWS.txt (CASSANDRA-14183)
 
diff --git a/build.xml b/build.xml
index 3ac2665..ce47717 100644
--- a/build.xml
+++ b/build.xml
@@ -1040,14 +1040,14 @@
 
     <target name="release" depends="artifacts,rat-init"
             description="Create and QC release artifacts">
-      <checksum forceOverwrite="yes" todir="${build.dir}" fileext=".md5"
-                algorithm="MD5">
+      <checksum forceOverwrite="yes" todir="${build.dir}" fileext=".sha256"
+                algorithm="SHA-256">
         <fileset dir="${build.dir}">
           <include name="*.tar.gz" />
         </fileset>
       </checksum>
-      <checksum forceOverwrite="yes" todir="${build.dir}" fileext=".sha"
-                algorithm="SHA">
+      <checksum forceOverwrite="yes" todir="${build.dir}" fileext=".sha512"
+                algorithm="SHA-512">
         <fileset dir="${build.dir}">
           <include name="*.tar.gz" />
         </fileset>