Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/ci.yml

@@ -14,7 +14,7 @@ jobs:
         xcode: ["14.2"]
     steps:
       - name: Checkout
-        uses: actions/checkout@v3.5.2
+        uses: actions/checkout@v6.0.1
 
       - name: Select Xcode ${{ matrix.xcode }}
         run: sudo xcode-select -s /Applications/Xcode_${{ matrix.xcode }}.app
@@ -38,7 +38,7 @@ jobs:
       image: swift:${{ matrix.swift }}
     steps:
       - name: Checkout
-        uses: actions/checkout@v3.5.2
+        uses: actions/checkout@v6.0.1
 
       - name: Restore APT cache
         uses: actions/cache@v3