Refactor KubernetesClient class

Context

The KubernetesClient class is too complex for what it's supposed to accomplish. It's a small technical debt I encounter every time I need to find a bug in it.

From Draft to Ready

• Merge !112658 (merged)
• !114270 (merged) is merged, and we rebased master onto this branch

What does this MR do?

This MR reduces the technical debt in this class by making it a bit more straightforward.

• Refactored the specs for the KubernetesClient class 🎉
• Makes scripts/review_apps/automated_cleanup.rb ready for helm3 (we also added specs for this class in !114270 (merged), so that we are more confident with this refactor)
• Split cleanup_by_created_at generic method into two more specific methods: cleanup_namespaces_by_created_at and cleanup_pvcs_by_created_at
• Fix a PVC deletion bug.
  • With helm 2, we created all the PVCs in the review-apps namespace. This has changed in Helm 3, and therefore, we didn't properly delete PVCs anymore in the cluster.
• Adds the possibility to run review-cleanup in dry-run mode by passing DRY_RUN to the manual job.

Does it work?

• The review-cleanup dry-run job was run in https://gitlab.com/gitlab-org/gitlab/-/jobs/3953335047

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.