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] <[email protected]>

Author: dependabot[bot]

.github/workflows/ci.yml

@@ -13,7 +13,7 @@ jobs:
         os: [macos-13, ubuntu-24.04]
     runs-on: ${{ matrix.os }}
     steps:
-        - uses: actions/checkout@v5
+        - uses: actions/checkout@v6
         - uses: fwal/setup-swift@v2
         - name: Echo Swift version
           run: swift --version
@@ -26,7 +26,7 @@ jobs:
         swift-version: ['5.10', '6.0', '6.1']
     runs-on: ubuntu-24.04
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - uses: fwal/setup-swift@v2
         with:
           swift-version: ${{ matrix.swift-version }}