Update GitHub Actions to use actions/checkout@v6 in multiple workflows

Author: jaydrogers

.github/workflows/action_update-dockerhub-readme.yml

@@ -14,7 +14,7 @@ jobs:
     name: Push README to Docker Hub
     steps:
       - name: git checkout
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           ref: main
 

.github/workflows/scheduled-task_update-sponsors.yml

@@ -8,7 +8,7 @@ jobs:
     runs-on: ubuntu-24.04
     steps:
       - name: Checkout 🛎️
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
 
       - name: Generate Sponsors 💖
         uses: JamesIves/github-sponsors-readme-action@v1

.github/workflows/service_docker-build-and-publish.yml

@@ -39,7 +39,7 @@ jobs:
       php-version-map-json: ${{ steps.get-php-versions.outputs.php-version-map-json }}
     steps:
     - name: Check out code
-      uses: actions/checkout@v5
+      uses: actions/checkout@v6
       with:
         ref: ${{ inputs.ref }}
 
@@ -80,7 +80,7 @@ jobs:
 
     steps:
       - name: Check out code.
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           ref: ${{ inputs.ref }}