ci: support java 21 virtual threads

Signed-off-by: Marcos Tischer Vallim <tischer@gmail.com>

Author: mvallim

.github/workflows/ci-maven.yml

@@ -34,7 +34,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        java-version: [8, 11, 17]
+        java-version: [8, 11, 17, 21]
     steps:
       - name: Checkout repository
         uses: actions/checkout@v4