Fixed release check polling to use required check only

The previous approach iterated all check runs and treated any
non-success as a blocker. This caused two issues:
- Self-referential loop: the release job itself appeared as in_progress
- Optional checks (e.g. SonarCloud) blocked the release on failure

Now polls only the "All required tests passed or skipped" check,
which is the CI gate that aggregates all mandatory checks. This
matches Ghost-Release's behavior.

ref https://linear.app/ghost/issue/BER-3313