Bump actions/checkout from 4 to 6

Bumps [actions/checkout](https://github.com/actions/checkout) from 4 to 6.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v4...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>

Author: dependabot[bot]

.github/workflows/publish_release.yml

@@ -62,7 +62,7 @@ jobs:
           echo "Using swift-syntax version: $SWIFT_SYNTAX_VERSION"
           echo "swift_syntax_version=$SWIFT_SYNTAX_VERSION" >> "$GITHUB_OUTPUT"
       - name: Checkout repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
       - name: Tag release
         run: |
           git tag "${{ steps.swift_syntax_version.outputs.swift_syntax_version }}"
@@ -77,7 +77,7 @@ jobs:
       contents: write
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
       - name: Create release
         env:
           GH_TOKEN: ${{ github.token }}

.github/workflows/pull_request.yml

@@ -30,7 +30,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
         with:
           persist-credentials: false
       - name: Update environment
@@ -47,7 +47,7 @@ jobs:
       image: swift:latest
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
         with:
           persist-credentials: false
       - name: Validate generated code
@@ -59,7 +59,7 @@ jobs:
       image: swift:latest
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
         with:
           persist-credentials: false
       - name: Test
@@ -75,7 +75,7 @@ jobs:
           docker pull $Image
           echo "image=$Image" >> "$env:GITHUB_OUTPUT"
       - name: Checkout repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
         with:
           persist-credentials: false
       - name: Test