Update actions/checkout action to v4

Author: renovate[bot]

.github/workflows/ci-linux.yml

@@ -17,7 +17,7 @@ jobs:
       image: swift:${{ matrix.swift }}
     steps:
       - name: Checkout
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
 
       - name: Test
         run: swift test --skip-update

.github/workflows/ci-macos.yml

@@ -15,7 +15,7 @@ jobs:
         xcode: ["13.4.1", "14.2"]
     steps:
       - name: Checkout
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
       - name: Select Xcode ${{ matrix.xcode }}
         run: sudo xcode-select -s /Applications/Xcode_${{ matrix.xcode }}.app
       - name: Test