ci: rerun contribution gate after PR updates

Include pull_request_target synchronize events so the required gate context is attached to the current head after contributors push follow-up commits.

Signed-off-by: hexin <he.xin@h3c.com>

Author: h3c-hexin

.github/workflows/pr-gate.yml

@@ -2,7 +2,7 @@ name: Contribution gate - pull requests
 
 on:
   pull_request_target:
-    types: [opened, reopened]
+    types: [opened, reopened, synchronize]
 
 permissions:
   contents: read