Bump actions/checkout from 5 to 6 (#5026)

Bumps [actions/checkout](https://github.com/actions/checkout) from 5 to 6.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](https://github.com/actions/checkout/compare/v5...v6)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

.github/workflows/check-whitespace.yml

@@ -16,7 +16,7 @@ jobs:
   check-whitespace:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
         with:
           fetch-depth: 0
 

.github/workflows/integrate.yaml

@@ -24,20 +24,20 @@ jobs:
 
     steps:
       - name: "Checkout php/doc-${{ matrix.language }}"
-        uses: "actions/checkout@v5"
+        uses: "actions/checkout@v6"
         with:
           path: "${{ matrix.language }}"
           repository: "php/doc-${{ matrix.language }}"
 
       - name: "Checkout php/doc-en as fallback"
         if: "matrix.language != 'en'"
-        uses: "actions/checkout@v5"
+        uses: "actions/checkout@v6"
         with:
           path: "en"
           repository: "php/doc-en"
 
       - name: "Checkout php/doc-base"
-        uses: "actions/checkout@v5"
+        uses: "actions/checkout@v6"
         with:
           path: "doc-base"
           repository: "php/doc-base"