automate repo cleanup

[emmyoop]

resolves dbt-labs/actions#78
docs dbt-labs/docs.getdbt.com/#

Problem

Automated release and branch cutting has cluttered the repo with stray branches and draft releases.

Solution

• Delete branches matching patterns from releasing and cutting new branches: branch: ["cutting_release_branch/*", "prep-release/*"]
• Delete releases in draft status

This will run every Saturday on a schedule and can also be manually submitted. Running on Saturday will prevent this from colliding with an actual release mid-processing since we do no cut new releases on the weekend.

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX