Use actions/checkout@v4 (#24)

Author: Rangi42

.github/workflows/testing.yml

@@ -12,7 +12,7 @@ jobs:
         rgbds: ['', '--HEAD']
     runs-on: macos-latest
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - name: Install RGBDS
         run: |
           brew install ${{ matrix.rgbds }} rgbds