Prepare bigint indexes for merge_requests stage two

What does this MR do and why?

Async index creation for column target_project_id, latest_merge_request_diff_id, last_edited_by_id.

"index_merge_requests_on_target_project_id_and_iid" UNIQUE, btree (target_project_id, iid)
"index_merge_requests_on_target_project_id_and_merged_commit_sha" btree (target_project_id, merged_commit_sha)
"index_merge_requests_on_target_project_id_and_source_branch" btree (target_project_id, source_branch)
"index_merge_requests_on_target_project_id_and_squash_commit_sha" btree (target_project_id, squash_commit_sha)
"index_merge_requests_on_target_project_id_and_target_branch" btree (target_project_id, target_branch) WHERE state_id = 1 AND merge_when_pipeline_succeeds = true
"index_merge_requests_on_latest_merge_request_diff_id" btree (latest_merge_request_diff_id)
"index_merge_requests_for_latest_diffs_with_state_merged" btree (latest_merge_request_diff_id, target_project_id) WHERE state_id = 3
-- 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_merge_requests_on_author_id_and_target_project_id" btree (author_id, target_project_id)
"index_on_merge_requests_for_latest_diffs" btree (target_project_id) INCLUDE (id, latest_merge_request_diff_id)

References

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

Screenshots or screen recordings

NA

How to set up and validate locally

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