Test with JDK 21-ea on GitHub Actions

Author: tbroyer

.github/workflows/maven.yml

@@ -78,7 +78,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        java: [ 11, 17, 18 ]
+        java: [ '11', '17', '21-ea' ]
     steps:
       - uses: actions/checkout@v3