switch to shared pr label enforcers, add bugfix only action

[alexrashed]

Motivation

We have several repositories where we want to enforce some label constraints on PRs.
This PR migrates these actions to shared workflows in https://github.com/localstack/meta.
These actions are only executed on newly created PRs. The actions will only be triggered if the labels on a PR are changed.
So we still have to watch out for PRs which have already been created!

Changes

• Switch to shared workflows for:
  • pr-enforce-no-major-master.yml
  • pr-enforce-pr-labels.yml
• Introduce a new workflow (also using a shared one) to enforce that neither major nor minor labels are on the PR: pr-enforce-no-major-minor-master.yml.

Testing

• These changes were tested in another private org repo.

TODO

• After merging these PRs we have to disable the pr-enforce-no-major-master.yml, such that only the pr-enforce-no-major-minor-master.yml is active (which enforces our current restriction on not merging minor changes until we release the next bugfix release).

[coveralls]

coverage: 84.006% (-0.002%) from 84.008%
when pulling dd0dd5a on use-shared-label-enforcers
into d1e54f4 on master.

[dominikschubert]

LGTM