Update actions/checkout action to v4

Author: renovate[bot]

.github/workflows/maven.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
 
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@3df4ab11eba7bda6032a0b82a6bb43b11571feac # v4
 
     - name: Set up JDK 15
       uses: actions/setup-java@v2