Pin actions/checkout to an exact SHA reference

See: https://docs.zizmor.sh/audits/#unpinned-uses

Author: mpkorstanje

.github/workflows/maven.yml

@@ -12,11 +12,11 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - name: Checkout code
-      uses: actions/checkout@v6
+      uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
       with:
         path: code
     - name: Checkout pages
-      uses: actions/checkout@v6
+      uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
       with:
         path: gh-pages
         ref: gh-pages