[ADAP-481] Add scheduled testing for release branches

### High Level Task
Add a new workflow to start daily automated tests.

### Acceptance Criteria
- [ ] `release-branch-tests.yml` runs on a schedule and posts to slack on failure

### Details
Add a new workflow similar to [`release-branch-tests.yml`](https://github.com/dbt-labs/dbt-bigquery/blob/main/.github/workflows/release-branch-tests.yml) in `dbt-bigquery`.

Note: As of today the adapters have [hard coded lists](https://github.com/dbt-labs/dbt-bigquery/blob/d2bef8672a395b1aba6a673d506f5a7ae6e9a483/.github/workflows/release-branch-tests.yml#L42) of release branches. This should be [dynamically determined like in core](https://github.com/dbt-labs/dbt-core/blob/c239149c7375d2a4cb08f03e40d71061aa678cb1/.github/workflows/release-branch-tests.yml#L31-L54).

### Open Questions
What schedule do we want to run these on? Snowflake runs less often than other adapters.