[CI] Fix backporting

[emmyoop]

Housekeeping

• I am a maintainer of dbt-core

Short description

Current backporting is very flaky.

Acceptance criteria

1. only team members should be able to backport
2. We should be able to auto-commit backports to the target branch and not have to go through the PR process and get reviews
3. You should be able to put the label on at any time before or after merge and the backport will happen after merge
4. If backport fails it should send notification - comment or preferably slack message

Suggested Tests

Manual testing by backporting to a fake branch

Impact to Other Teams

none

Will backports be required?

no

Context

No response