Add add_concurrent_index migration for index_users_on_updated_at

What does this MR do and why?

Addresses the performance issues raised in #392019 (closed) by adding a post migration to create a new index_users_on_updated_at index with add_concurrent_index. This was originally implemented as a scheduled asynchronous creation as per the adding database indexes guide, but after assessing the necessary time to create the index was just a few minutes, it was decided that the overhead of asynchronous creation was not worth it.

Screenshots or screen recordings

Migrate:

main: == 20230218145930 AddIndexUsersOnUpdatedAt: migrating =========================
main: -- transaction_open?()
main:    -> 0.0000s
main: -- view_exists?(:postgres_partitions)
main:    -> 0.1349s
main: -- index_exists?("users", :updated_at, {:name=>"index_users_on_updated_at", :algorithm=>:concurrently})
main:    -> 0.0311s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0003s
main: -- add_index("users", :updated_at, {:name=>"index_users_on_updated_at", :algorithm=>:concurrently})
main:    -> 0.0093s
main: -- execute("RESET statement_timeout")
main:    -> 0.0004s
main: == 20230218145930 AddIndexUsersOnUpdatedAt: migrated (0.1970s) ================

ci: == 20230218145930 AddIndexUsersOnUpdatedAt: migrating =========================
ci: -- transaction_open?()
ci:    -> 0.0000s
ci: -- view_exists?(:postgres_partitions)
ci:    -> 0.0008s
ci: -- index_exists?("users", :updated_at, {:name=>"index_users_on_updated_at", :algorithm=>:concurrently})
ci:    -> 0.0124s
ci: -- execute("SET statement_timeout TO 0")
ci:    -> 0.0002s
ci: -- add_index("users", :updated_at, {:name=>"index_users_on_updated_at", :algorithm=>:concurrently})
ci:    -> 0.0045s
ci: -- execute("RESET statement_timeout")
ci:    -> 0.0002s
ci: == 20230218145930 AddIndexUsersOnUpdatedAt: migrated (0.0326s) ================

Creating index with Ask Joe bot:

Query performance improvements

Before adding the index:

Time: 3.252 min  
  - planning: 2.717 ms  
  - execution: 3.252 min  
    - I/O read: 9.500 min  
    - I/O write: 1.057 s  
  
Shared buffers:  
  - hits: 64 (~512.00 KiB) from the buffer pool  
  - reads: 798284 (~6.10 GiB) from the OS file cache, including disk I/O  
  - dirtied: 54979 (~429.50 MiB)  
  - writes: 25031 (~195.60 MiB)

Postgres.ai explain: https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/15635/commands/54083

After adding the index:

Time: 4.145 ms  
  - planning: 3.693 ms  
  - execution: 0.452 ms  
    - I/O read: 0.333 ms  
    - I/O write: 0.000 ms  
  
Shared buffers:  
  - hits: 0 from the buffer pool  
  - reads: 4 (~32.00 KiB) from the OS file cache, including disk I/O  
  - dirtied: 0  
  - writes: 0

Postgres.ai explain: https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/15635/commands/54087

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.