ci: add Java 21 to the test matrix

Author: NickDub

.github/workflows/test.yml

@@ -21,7 +21,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest, windows-latest, macos-latest]
-        java-version: [8, 11, 17]
+        java-version: [8, 11, 17, 21]
     runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v4