Kubernetes deployments on new clusters will now have
a separate namespace per project environment, instead
of sharing a single namespace for the project.

Behaviour of existing clusters is unchanged.

All new functionality is controlled by the
:kubernetes_namespace_per_environment feature flag,
which is safe to enable/disable at any time.

Also removes all logic from ClusterConfigureWorker
and ClusterProjectConfigureWorker, which are also
no longer used.

- Set as uninstallable app
- Update docs
- Adjust specs

Resolve "API support for group-level clusters"

Closes #55623

See merge request gitlab-org/gitlab-ce!30213

GitLab uses a kubernetes service account to perform deployments. For
serverless deployments to work as expected with externally created
clusters with their own knative installations (e.g. via Cloud Run), this
account requires additional permissions in the serving.knative.dev API
group.

The current approach requires catching exceptions to handle these errors
and callers are already handling model validations so it seems more
appropriate.  Also it seemed to convoluted to add this logic directly to
the model since the model needs to check too many possible associations
to determine whether or not there are more than one cluster since the
model doesn't know what it's being created on. Additionally we only
wanted to validate during create to avoid the risk of existing models
becoming invalid by many different edge cases.

Enable JIT Kubernetes resource creation for project level clusters

See merge request gitlab-org/gitlab-ce!29515

Since Kubernetes is creating the Secret and token asynchronously it is
necessary that we implement some delay or retrying logic to avoid a race
condition where we fetch a Secret before the token is even set. There
does not appear to be any way for us to force it to be set with any
synchronous API call so retrying seems to be the only option.

Previously this behaviour was only available to group
and instance-level clusters, as some project clusters
relied on Kubernetes credentials being passed through
to the runner instead of having their resources managed
by GitLab (which is not available when using JIT). These
clusters have been migrated to unmanaged, so resources
can be created on demand for the remaining managed clusters.