CI: Update to actions/checkout@v3.

.github/workflows/msvc.yml

@@ -23,7 +23,7 @@ jobs:
             project: VisualC-WinRT/SDL-UWP.sln, projectflags: '/p:Platform=x64 /p:WindowsTargetPlatformVersion=10.0.17763.0' }
 
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v3
     - name: Create CMake project using SDL as a subproject
       shell: python
       run: |