CI: Update to actions/checkout@v3.

.github/workflows/ios.yml

@@ -15,6 +15,6 @@ jobs:
         - { name: tvOS, target: Static Library-tvOS, sdk: appletvos }
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
       - name: Build
         run: xcodebuild -project Xcode/SDL/SDL.xcodeproj -target '${{ matrix.platform.target }}' -configuration Release -sdk ${{ matrix.platform.sdk }} clean build