Add Java 21 build

Update to checkout@v4

Author: stefanseifert

.github/workflows/maven-build.yml

@@ -19,13 +19,13 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        java: [11, 17]
+        java: [11, 17, 21]
         os: [ubuntu-latest]
         distribution: [temurin]
 
     steps:
       - name: Checkout code
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
 
       - name: Setup JDK
         uses: actions/setup-java@v3