CI build: remove JDK 11 build, switch 21 to 21-ea

Background: JDT Core dependencies now contain hundreds of Java 17 class
files, i.e. Ajc now requires Java 17+ runtime environment. :-(
For more details, see:
https://github.com/eclipse-aspectj/aspectj/issues/260#issuecomment-1736540176

Adoptium Java 21 release is not available yet, so we need to run on EA.

Signed-off-by: Alexander Kriegisch <Alexander@Kriegisch.name>

.github/workflows/maven.yml

       fail-fast: false
       matrix:
         # Check for available Temurin releases on https://adoptium.net/releases.html
-        java: [ 11, 17, 21 ]
+        java: [ 17, 21-ea ]
 
     runs-on: ubuntu-latest