Upgrade to actions/checkout@v6 to fix Actions warning

Author: nk9

.github/workflows/syntax-tests.yml

@@ -28,7 +28,7 @@ jobs:
             packages: master
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v6
       - uses: SublimeText/syntax-test-action@v2
         with:
           build: ${{ matrix.build }}