- Add test for N+1 queries
- Add destroyable lists scope to Board and List
- Preload lists for both project and group boards

It also renames the API::PROJECT_ENDPOINT_REQUIREMENTS constant to
API::NAMESPACE_OR_PROJECT_REQUIREMENTS

Partially addresses #47424.

Had to make changes to spec files because
stubbing methods on frozen objects is a mess
in RSpec and leads to failures:

https://github.com/rspec/rspec-mocks/issues/1190

Allow Labels::FindOrCreateService to find ancestor group labels
Add authentication check on API endpoint
Update flayignore

Signed-off-by: Rémy Coutable <remy@rymai.me>

  - Includes documentation and tests