Skip label removal/readd if PR description changed by type didn't

[ArthurSens]

In our current auto-label workflow, when a PR is edited, we first remove the changelog-related label and after that we re-add them.

We see two problems with this:

• If a PR description changes but /kind doesn't, we're doing unnecessary work when we remove and re-add the same thing.
• We've been noticing some race conditions (not sure where exactly). Sometimes the label is removed but is not re-added again, causing the workflow to fail.