ci: use actions/checkout@v3 instead of @v2

Author: icculus

.github/workflows/main.yml

@@ -19,7 +19,7 @@ jobs:
       with:
         version: sdl2-latest
     - name: Get MojoAL sources
-      uses: actions/checkout@v2
+      uses: actions/checkout@v3
     - name: Configure CMake
       run: cmake -B build ${{ matrix.platform.flags }}
     - name: Build