Bump actions/checkout from 5 to 6

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](actions/checkout@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>

Author: dependabot[bot]

.github/workflows/integration-test.yaml

@@ -23,7 +23,7 @@ jobs:
         env:
           SWIFTLY_GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
       - name: Checkout
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
       - name: Resolve Swift dependencies
         run: swift package resolve
         working-directory: ./IntegrationTests

.github/workflows/soundness.yaml

@@ -11,7 +11,7 @@ jobs:
     timeout-minutes: 1
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
       - name: Run unacceptable language check
         run: ./scripts/check-unacceptable-language.sh
 
@@ -21,7 +21,7 @@ jobs:
     timeout-minutes: 1
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
       - name: Run license header check
         run: ./scripts/check-license-headers.sh
 
@@ -31,7 +31,7 @@ jobs:
     timeout-minutes: 5
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
       - name: Install Swift
         uses: vapor/swiftly-action@v0.2
         with:
@@ -47,7 +47,7 @@ jobs:
     timeout-minutes: 1
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           submodules: true
       - name: Run broken symlinks check

.github/workflows/unit-test.yaml

@@ -23,7 +23,7 @@ jobs:
         env:
           SWIFTLY_GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
       - name: Checkout
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
       - name: Resolve Swift dependencies
         run: swift package resolve
       - name: Run Unit Tests