refactor(handlers): use single static handler for GitLab V1 base route

Related to: #866 (closed)

This MR refactors conditionally returning 404s for the GitLab V1 base route when the database is disabled. This step helps ensure that we have a smooth path towards a unified chi router as part of Replace the Router used for API Requests (&9467). We want to move away from route dispatching at runtime, and this step ensures that we're functional with the old router before moving to chi.