Use checkout action v4 (#19)

Author: adrenak

.github/workflows/upm-branch-sync.yml

@@ -8,7 +8,7 @@ jobs:
     name: Sync UPM branch
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
       with:
         fetch-depth: 0