Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/gradle.yml

@@ -13,7 +13,7 @@ jobs:
     timeout-minutes: 15
 
     steps:
-    - uses: actions/checkout@08eba0b27e820071cde6df949e0beb9ba4906955 # v4
+    - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6
 
     - name: Set up JDK 17
       uses: actions/setup-java@dded0888837ed1f317902acf8a20df0ad188d165 # v5