Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/ci-linux.yml

@@ -16,7 +16,7 @@ jobs:
       image: swift:${{ matrix.swift }}
     steps:
       - name: Checkout
-        uses: actions/checkout@v3.6.0
+        uses: actions/checkout@v6.0.0
 
       - name: Restore APT cache
         uses: actions/[email protected]

.github/workflows/ci-macos.yml

@@ -14,7 +14,7 @@ jobs:
         xcode: ["14.3.1", "15.0"]
     steps:
       - name: Checkout
-        uses: actions/checkout@v3.6.0
+        uses: actions/checkout@v6.0.0
       - name: Select Xcode ${{ matrix.xcode }}
         run: sudo xcode-select -s /Applications/Xcode_${{ matrix.xcode }}.app
       - name: Restore cached Homebrew dependencies