ci: adds Java 21 (LTS) to CI test targets.

Author: outofcoffee

.github/workflows/validate.yaml

@@ -8,7 +8,7 @@ jobs:
     timeout-minutes: 15
     strategy:
       matrix:
-        java: [ '11', '17' ]
+        java: [ '11', '17', '21' ]
     permissions:
       checks: write
     env: