Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/swift.yml

@@ -12,7 +12,7 @@ jobs:
   build:
     runs-on: macos-latest
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - name: Install Xcode 26.0
       run: sudo xcode-select -s /Applications/Xcode_26.0.app
     - name: Install swiftlint