In https://gitlab.com/gitlab-org/gitlab/-/merge_requests/74800, 2
indexes were added to `merge_request_assignees` and
`merge_request_reviewers` tables to improve the performance of the
finder query for attention requests.

In https://gitlab.com/gitlab-org/gitlab/-/merge_requests/80732, the
finder query was used to show the count of attention requests.

While investigating a bug related to the count, found out that the
count query isn't performant enough. Based on this query plan
(https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/9497/commands/33700),
it takes around 447ms.

It's noticeable that the query plan isn't using the index added on
the first MR when querying the `merge_request_assignees` and
`merge_request_reviewers` tables.

To fix this, narrower indexes are added on both tables for `user_id`
and `state` columns. This also deletes the `state` indexes that
were added before. The query timing dropped to 14ms from 447ms.

Query plan: https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/9497/commands/33717

Changelog: performance