Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/gradle-build.yml

@@ -5,7 +5,7 @@ jobs:
     name: "Building"
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - uses: gradle/actions/wrapper-validation@v5
     - name: "Set up JDK 11"
       uses: actions/setup-java@v5

.github/workflows/mkdocs-deploy.yml

@@ -10,7 +10,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout master
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
 
       - name: Deploy docs
         uses: mhausenblas/mkdocs-deploy-gh-pages@1.26