Add synchronize event to the hooks of "Check Required Labels"

[kyu08]

PR Description

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.

Actually, #4936 is green in spite of no label is added.

Please check if the PR fulfills these requirements

• Cheatsheets are up-to-date (run go generate ./...)
• Code has been formatted (see here)
• Tests have been added/updated (see here for the integration test guide)
• Text is internationalised (see here)
• If a new UserConfig entry was added, make sure it can be hot-reloaded (see here)
• Docs have been updated if necessary
• You've read through your own file changes for silly mistakes etc

Testing

I tested this change fixes the problem I mentioned using my fork repo.

What I did is:

1. Push this commit to master branch of my fork repository. It's almost same as this PR's change.
2. Create a PR bases to master.
3. Push new commit to master branch.
4. Press "Update branch" in the PR.
5. Ensure that PR status become red as expected.

[stefanhaller]

LGTM. Why is it marked as draft though?

[kyu08]

Thanks for the review!
Simply I forgot mark as opened. 😅