Make k8s namespace cleanup method public

What does this MR do and why?

Fix tiny bug introduced in !112532 (merged).

This MR fixes errors as seen in https://gitlab.com/gitlab-org/gitlab/-/jobs/3811165709,

scripts/review_apps/automated_cleanup.rb:246:in `delete_helm_releases': private method `delete_namespaces_by_exact_names' called for #<Tooling::KubernetesClient:0x00007f9c53ad0dd0 @namespace="review-apps"> (NoMethodError)
	from scripts/review_apps/automated_cleanup.rb:161:in `perform_helm_releases_cleanup!'
	from scripts/review_apps/automated_cleanup.rb:304:in `block in <main>'
	from scripts/review_apps/automated_cleanup.rb:275:in `timed'
	from scripts/review_apps/automated_cleanup.rb:303:in `<main>'

This call happens when we need to clean up helm releases, which is once a day.

We'll add specs to this class in #392858 (closed).

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