This reverts merge request !27514

remove `Clusters::Platforms::Kubernetes#actual_namespace`

Closes #61935

See merge request gitlab-org/gitlab-ce!28391

Reorganize test jobs by level

See merge request gitlab-org/gitlab-ce!27514

We kept it for smooth update only

Files in lib will be eager loaded and hence will require haml_lint to be
loaded. Since this is only a development dependency, we can't assume
this gem will be available in production, so it should never be loaded
in production.

The migration pod for the GitLab Helm Chart was failing because
of the NameError:

uninitialized constant HamlLint::Linter::NoPlainNodes::LinterRegistry

We now explicitly specify ::HamlLint::LinterRegistry to avoid resolution
errors.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/62125

When Kubernetes clusters were originally built they could only
exist at the project level, and so there was logic included
that assumed there would only ever be a single Kubernetes
namespace per cluster. We now support clusters at the group
and instance level, which allows multiple namespaces.

This change consolidates various project-specific fallbacks to
generate namespaces, and hands all responsibility to the
Clusters::KubernetesNamespace model. There is now no concept of
a single namespace for a Clusters::Platforms::Kubernetes; to
retrieve a namespace a project must now be supplied in all cases.

This simplifies upcoming work to use a separate Kubernetes
namespace per project environment (instead of a namespace
per project).

Add HamlLint::Linter::NoPlainNodes linter

See merge request gitlab-org/gitlab-ce!28265

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/50813