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] <[email protected]>

Author: dependabot[bot]

.github/workflows/ci.yml

@@ -11,7 +11,7 @@ jobs:
     name: macOS (Xcode 16.0, Swift 6.0)
     runs-on: macos-15
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - name: Select Xcode 16.0
         run: sudo xcode-select -s /Applications/Xcode_16.0.app
       - name: Swift version
@@ -25,7 +25,7 @@ jobs:
     name: macOS (Xcode 16.2, Swift 6.2)
     runs-on: macos-15
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - name: Select Xcode 16.2
         run: sudo xcode-select -s /Applications/Xcode_16.2.app
       - name: Swift version
@@ -40,7 +40,7 @@ jobs:
     runs-on: ubuntu-latest
     container: swift:6.0
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - name: Swift version
         run: swift --version
       - name: Build
@@ -52,7 +52,7 @@ jobs:
     name: Windows (Swift 6.0)
     runs-on: windows-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - uses: compnerd/gha-setup-swift@main
         with:
           branch: swift-6.0-release

.github/workflows/swift-format.yml

@@ -11,7 +11,7 @@ jobs:
     name: Swift Format Check
     runs-on: macos-15
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - name: Select Xcode 16.0
         run: sudo xcode-select -s /Applications/Xcode_16.0.app
       - name: Install swift-format

.github/workflows/swiftlint.yml

@@ -11,7 +11,7 @@ jobs:
     name: SwiftLint Check
     runs-on: macos-15
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - name: Install SwiftLint
         run: brew install swiftlint
       - name: Run SwiftLint