chore(alerts): Replace `pr_created` trigger with `pr_ready_for_review` (again) (#123055)

Revert revert of  https://github.com/getsentry/sentry/pull/122915.

Requires https://github.com/getsentry/getsentry/pull/21707 to be merged,
will be running the full test-suite to make sure this is green before
merging again 🙃