let_it_be for spec/controllers

What does this MR do?

Replaces set in spec/controllers with let_it_be

set has not been updated since 2014, and has some gotchas. So we want to migrate away from set in favor of let_it_be incrementally.

Part of #27922 (closed)

Timings

set	let_it_be
3m10.286s	3m14.774s
3m11.384s	3m18.986s
3m13.387s	3m13.877s

There seems to be no significant performance difference or regression, which is good

rspec command

``` time bin/rspec spec/controllers/admin/clusters_controller_spec.rb spec/controllers/admin/requests_profiles_controller_spec.rb spec/controllers/admin/users_controller_spec.rb spec/controllers/groups/clusters_controller_spec.rb spec/controllers/groups/labels_controller_spec.rb spec/controllers/groups/shared_projects_controller_spec.rb spec/controllers/import/bitbucket_server_controller_spec.rb spec/controllers/import/gitlab_projects_controller_spec.rb spec/controllers/projects/artifacts_controller_spec.rb spec/controllers/projects/autocomplete_sources_controller_spec.rb spec/controllers/projects/clusters_controller_spec.rb spec/controllers/projects/commit_controller_spec.rb spec/controllers/projects/environments/prometheus_api_controller_spec.rb spec/controllers/projects/environments_controller_spec.rb spec/controllers/projects/error_tracking_controller_spec.rb spec/controllers/projects/mirrors_controller_spec.rb spec/controllers/projects/pipeline_schedules_controller_spec.rb spec/controllers/projects/pipelines_controller_spec.rb spec/controllers/projects/pipelines_settings_controller_spec.rb spec/controllers/projects/settings/ci_cd_controller_spec.rb spec/controllers/projects/settings/operations_controller_spec.rb spec/controllers/projects/wikis_controller_spec.rb spec/controllers/projects_controller_spec.rb spec/controllers/search_controller_spec.rb ```

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team