Prepare bigint indexes for merge_requests stage three

What does this MR do and why?

Async index creation for column id, author_id.

"merge_requests_pkey" PRIMARY KEY, btree (id)
"idx_merge_requests_on_id_and_merge_jid" btree (id, merge_jid) WHERE merge_jid IS NOT NULL AND state_id = 4
"idx_merge_requests_on_merged_state" btree (id) WHERE state_id = 3
"idx_merge_requests_on_unmerged_state_id" btree (id) WHERE state_id <> 3
"index_merge_requests_on_author_id_and_id" btree (author_id, id)
"index_merge_requests_on_author_id_and_created_at" btree (author_id, created_at)
-- Following indexes are recreated due to other column migration
"idx_mrs_on_target_id_and_created_at_and_state_id" btree (target_project_id, state_id, created_at, id)
"index_merge_requests_on_target_project_id_and_created_at_and_id" btree (target_project_id, created_at, id)
"index_merge_requests_on_target_project_id_and_updated_at_and_id" btree (target_project_id, updated_at, id)
"index_merge_requests_on_tp_id_and_merge_commit_sha_and_id" btree (target_project_id, merge_commit_sha, id)
"index_on_merge_requests_for_latest_diffs" btree (target_project_id) INCLUDE (id, latest_merge_request_diff_id)
"index_merge_requests_on_author_id_and_target_project_id" btree (author_id, target_project_id)

References

Issue - Swap MergeRequest ID columns (Step 2) (#507695 - closed)

Screenshots or screen recordings

NA

How to set up and validate locally

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