Delete release testing branches during post-release workflow

[jbordonado]

Description

When running the post-release workflow, which marks the end of a release, we should also clean up the related testing/x.y.z-n branches instead of keeping them around. When creating those branches, we also add a tag so it's not a problem to find the associated code later, if needed.

Acceptance criteria

• First requirement
• Second requirement
• Third requirement
• etc.

Designs

Testing instructions

1. Go to '…'
2. Click on '…'
3. Scroll down to '…'
4. Observe expected behaviour

Dev notes

Additional context