Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/ci.yml

@@ -12,7 +12,7 @@ jobs:
     timeout-minutes: 15
 
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6
     - name: Set up JDK 17
       uses: actions/setup-java@v4
       with: