Roadmap epics limit improvements

While reviewing !47884 (merged), I noticed that there were a few odd things that we were doing.

1. We're counting unfiltered epics in https://gitlab.com/gitlab-org/gitlab/blob/f9cb0bf944ddcaf78632823e895a0ec0ef848f71/ee/app/controllers/groups/roadmap_controller.rb#L24. This is now only used for showing the empty state because the warning banner was changed so that it now looks at the count of the GraphQL response for epics.

   We also actually handle the empty state in the Vue app already so we could probably just remove this.

2. Max page size for epics is 2,000 and it seems like we're fetching 2,000 epics from the backend even though we say the limit is 1,000. Maybe we should also use gon.roadmap_epics_limit when making the query and fetch only 1,001 epics? (We need the extra one to know if we should render the warning banner or not). UPDATE: this task is tracked seprately: !52154 (merged)

These could be separate issues but I created this combined one because another option could actually be just fixing @epics_count so that it actually counts filtered epics, then our HAML warning banner would already be correct. We already moved the banner to Vue with that MR though so probably does not make sense to move it back. But I think we should definitely address fetching of unnecessary epics.

cc @fguibert @johnhope @felipe_artur