release: replace branch cut announce with release status
Created by: bobheadxi
Closes https://github.com/sourcegraph/sourcegraph/issues/13869 with a possible alternative:
- Replaces "branch cut is happening soon" with a more general informational update on release blockers and open issues
- Adds a step to post release status at release candidate time
Based on https://github.com/sourcegraph/sourcegraph/pull/14625, at which point the release status command posts a message with a count of release blockers and open issues.