kubevirt, labels: rename code-quality into cleanup

[dhiller]

What this PR does / why we need it:

The label sig/code-quality is used to mark pull requests that target improvements to our code base1. However, the prefix sig gives the impression that there is a SIG that is actively caring for such PRs.

Given the fact that there's neither an entry inside sigs.yaml2, nor a proper sig folder in the community repository, nor any publicly held meetings that are announced in the community calendar3, we can assume that such a sig doesn't exist.

During the discussion at the community meeting4 about this topic there was the suggestion to just rename the label into a kind label. Looking at Kubernetes labels provided a general label kind/cleanup5 that matches a broader set of scenarios and seems to be well suited for covering all the code-quality PRs.

Therefore this PR renames the label sig/code-quality to kind/cleanup.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged): Fixes #

Special notes for your reviewer: