fix: Don't merge transifex bot pull requests unless it passes the tests

[OmarIthawi]

Description

Now we have the validate-translation-files.yml workflow we should block the Transifex Bot from merging broken translations.

This should be configured in the GitHub repository settings even if it causes a backlog of pull requests to keep waiting.

Example of unwanted pull requests:

• Updates for file translations/frontend-app-course-authoring/src/i18n/transifex_input.json in fr_CA on branch main #1001 (comment) shouldn't be merged unless tests passes.

[OmarIthawi]

@brian-smith-tcril I've assigned you since I'm not an admin to the repository.

[brian-smith-tcril]

I added validate-po-files as a required status check for the branch protection rules on main. I'll leave this open until we verify it's working as intended.