Update the setup for configure a Java 21 JRE

- Also configure branches for PRs to rebase.

Author: merks

setup/m2e.setup

@@ -46,9 +46,8 @@
   </setupTask>
   <setupTask
       xsi:type="jdt:JRETask"
-      version="JavaSE-17"
-      location="${jre.location-17}"
-      name="JRE for JavaSE-17">
+      version="JavaSE-21"
+      location="${jre.location-21}">
     <description>Define the JRE needed to compile and run the Java projects of ${scope.project.label}</description>
   </setupTask>
   <setupTask
@@ -115,6 +114,12 @@
         <value>remoteURI</value>
       </detail>
     </annotation>
+    <configSections
+        name="branch">
+      <properties
+          key="autoSetupRebase"
+          value="always"/>
+    </configSections>
     <description>${scope.project.label}</description>
   </setupTask>
   <setupTask