This does not support upgrading from earlier versions

As mentioned in https://github.com/rails/rails/pull/29619, this removes
this warning message:

```
Delegating join_sources to arel is deprecated and will be removed in
Rails 6.0. (called from project_clusters_base_query at
app/models/clusters/clusters_hierarchy.rb:62)
```

Part of https://gitlab.com/gitlab-org/gitlab-ce/issues/61451

- refactor Knative and Prometheus

Calling #present? was causing a DB query to happen each time around the
loop. We only wanted to check for nil as it's nil in the first loop
around so there is no need for #present?

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.

- Closes #60024

- Change PrometheusClient.new to accept a base url instead of an
  already created RestClient

- Use Gitlab::HTTP in PrometheusClient instead of creating RestClient
  in PrometheusService

- Move http_options from PrometheusService to
  PrometheusClient (follow_redirects: false)

- ensure that base urls don't have the trailing slash

- Created a `PrometheusClient#url` method that might not be strictly
  required

- Change rescued exceptions from RestClient::* to
  HTTParty::ResponseError where possible and StandardError for the
  rest

Our current version of Cert-Manager does not uninstall
cleanly, and we must manually remove custom resource
definitions.