Prefer `json_response` where applicable.

Resolve "API support for group-level clusters"

Closes #55623

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

Banzai - avoid redis if attr is in DB cache

See merge request gitlab-org/gitlab-ce!30334

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.

When cache_collection_render runs we end up reading and writing
things to redis even if we already have the rendered field cached
in the DB. This commit avoids using redis at all whenever we have
the field already rendered in the DB cache.

https://gitlab.com/gitlab-org/gitlab-ce/issues/62826

rename `discussion` to `thread`

Closes #55953

See merge request gitlab-org/gitlab-ce!29553

Use title and description fields for issue trackers

Closes #63690

See merge request gitlab-org/gitlab-ce!30096