Add JDK 21 to CI

Author: cowtowncoder

.github/workflows/main.yml

@@ -24,7 +24,7 @@ jobs:
       fail-fast: false
       matrix:
         # 26-Sep-2025, tatu: some test failures on 21 cannot yet enable
-        java_version: ['8', '11', '17']
+        java_version: ['8', '11', '17', '21']
     env:
       JAVA_OPTS: "-XX:+TieredCompilation -XX:TieredStopAtLevel=1"
     steps: