upgrade CI Java version to 21

Signed-off-by: cesium147 <3448068+cesium147@users.noreply.github.com>

Author: cesium147

.github/workflows/CI.yml

@@ -13,7 +13,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest]
-        java: [17]
+        java: [17,21]
     name: Build and Test Plugin Template
     runs-on: ${{ matrix.os }}
     permissions: