ci: bump actions/checkout from 3 to 4 (#1281)

Author: dependabot[bot]

.github/workflows/check-merge-conflicts.yml

@@ -9,7 +9,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout code
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
         with:
           fetch-depth: 0