add more versions

Author: rheimus

.github/workflows/serversync-build.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        java: ["17"]
+        java: ["9", "17", "21"]
     name: Java ${{ matrix.java }} Build
     steps:
       - uses: actions/checkout@v4