Enable JIT Kubernetes resource creation for project level clusters

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

Project-level clusters that made use of this legacy
behaviour have been migrated to unmanaged clusters,
so we are now free to remove this fallback.

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.

Allow switching clusters between managed/unmanaged

Closes #60617

See merge request gitlab-org/gitlab-ce!29322

Migrate Kubernetes service integration templates to clusters

See merge request gitlab-org/gitlab-ce!28534

Any resources created while the cluster was managed
will be untouched, allowing users to set/unset this
flag as they please.

Whenever we are selecting a namespace to use for a
deployment or to query a cluster we want to exclude
Kubernetes namespace records that don't have a token
set as they will not have the required permissions.
However when configuring clusters, we want to
use the original namespace record even if it has no
token, as a namespace has to be unique on a cluster.

Clusters::Cluster::KnativeServicesFinder becomes
Clusters::KnativeServicesFinder

This is to avoid loading race condition between:

```
module Clusters
  class Cluster < ApplicationRecord
```

and

```
module Clusters
  class Cluster
```