Add synchronize event to the hooks of "Check Required Labels" (#4974)

I found that PR status becomes green even if no label added when we use
`Update branch` on GitHub PR page.

So I added `synchronize` event to the hooks of `Check Required Labels`.

Author: stefanhaller

.github/workflows/check-required-label.yml

@@ -2,7 +2,7 @@ name: Check Required Labels
 
 on:
   pull_request:
-    types: [opened, labeled, unlabeled]
+    types: [opened, labeled, unlabeled, synchronize]
 
 jobs:
   check-required-label: