all additional linter checks now optional

[Patrick Ohly]

Hello!

When introducing additional linter checks[1], we added a set of checks
which were considered too minor to justify sweeping changes in the
entire code base, but important enough to require fixing in new or
updated code. The merge-blocking pull-kubernetes-verify-lint job
enforced that.

In practice, this didn't work as well as hoped:
- It forced developers touching code which has such issues to address
them, whether it makes sense in their PR or not.
- Reverting a PR which fixed such an issue as part of some other change
failed in pull-kubernetes-verify-lint because the revert is treated
like "updated code" and re-introduces the issue.
- It's not entirely clear which issues fall into this "intermediate severity".

Therefore pull-kubernetes-verify-lint got removed again[2]. The
optional pull-kubernetes-linter-hints remains. From now on, it's
entirely up to the discretion of code authors and reviewers which of the
issues reported by pull-kubernetes-linter-hints they want to fix in a
pull request.

The intermediate severity of checks in Kubernetes (= hack/golangci-strict.yaml)
will get removed after the code freeze is over. Speaking of code freeze,
there are two special cases that you may encounter right now:

- For PRs where pull-kubernetes-verify-lint ran earlier, it's now listed
as "Waiting for status to be reported". This is safe to ignore. The
tide status should list "All tests passed", so this won't block
merging.

- If pull-kubernetes-linter-hints reports issues and the PR gets LGTM
and approval because they don't need to be fixed, the bot will
keep retesting pull-kubernetes-linter-hints. A manual /hold until code
freeze is over will prevent that.

[1] https://groups.google.com/a/kubernetes.io/g/dev/c/myGiml72IbM/m/jzaFiYxJAgAJ
[2] https://github.com/kubernetes/test-infra/pull/33857

--
Best Regards

Patrick Ohly
Cloud Software Architect