[CE] Upgrade to Rails 5.2

See merge request gitlab-org/gitlab-ce!30052

Resolve "API support for group-level clusters"

Closes #55623

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

Updates changed method names and fixes spec failures

This is basically a copy of the API for project clusters.

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.

Suggests to use a JSON structured log instead

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/54102

Rename ProjectEntity to IssuableEntity

Closes #58275

See merge request gitlab-org/gitlab-ce!30217

- Remove `FeatureConstrainer` call wrapping api endpoint
- Remove `Feature.enabled?(:graphql)` conditionals in back and frontend
- Modify graphql test to be graphql flag agnostic
- Remove api routing spec
- Remove frontend feature flag via `gon`