CI Build: build with LTS JDK 21

JDK 24 will come later as it is not supported by gradle yet (maybe in 8.14)

Signed-off-by: Christoph Rueger <chrisrueger@gmail.com>

Update cibuild.yml

Author: chrisrueger

.github/workflows/cibuild.yml

@@ -45,7 +45,7 @@ jobs:
         os:
         - 'ubuntu-latest'
         java:
-        - '20'
+        - '21'
         runner:
         - '{0}' # 'xvfb-run --auto-servernum {0}'
         include: