enable java 21 testing in CI (#4130)

Author: pjfanning

.github/workflows/main.yml

@@ -25,7 +25,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        java_version: ['8', '11', '17']
+        java_version: ['8', '11', '17', '21']
         os: ['ubuntu-20.04']
     env:
       JAVA_OPTS: "-XX:+TieredCompilation -XX:TieredStopAtLevel=1"