Add add_concurrent_index migration for index_namespaces_on_updated_at

What does this MR do and why?

Addresses the performance issues raised in #392020 (closed) by adding a post migration to create a new index_namespaces_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: == 20230218145940 AddIndexNamespacesOnUpdatedAt: migrating ====================
main: -- transaction_open?()
main:    -> 0.0000s
main: -- view_exists?(:postgres_partitions)
main:    -> 0.1772s
main: -- index_exists?("namespaces", :updated_at, {:name=>"index_namespaces_on_updated_at", :algorithm=>:concurrently})
main:    -> 0.0087s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0001s
main: -- add_index("namespaces", :updated_at, {:name=>"index_namespaces_on_updated_at", :algorithm=>:concurrently})
main:    -> 0.0045s
main: -- execute("RESET statement_timeout")
main:    -> 0.0002s
main: == 20230218145940 AddIndexNamespacesOnUpdatedAt: migrated (0.2069s) ===========

ci: == 20230218145940 AddIndexNamespacesOnUpdatedAt: migrating ====================
ci: -- transaction_open?()
ci:    -> 0.0000s
ci: -- view_exists?(:postgres_partitions)
ci:    -> 0.0007s
ci: -- index_exists?("namespaces", :updated_at, {:name=>"index_namespaces_on_updated_at", :algorithm=>:concurrently})
ci:    -> 0.0139s
ci: -- execute("SET statement_timeout TO 0")
ci:    -> 0.0002s
ci: -- add_index("namespaces", :updated_at, {:name=>"index_namespaces_on_updated_at", :algorithm=>:concurrently})
ci:    -> 0.0033s
ci: -- execute("RESET statement_timeout")
ci:    -> 0.0003s
ci: == 20230218145940 AddIndexNamespacesOnUpdatedAt: migrated (0.0332s) ===========

Creating index with Ask Joe bot:

Query performance improvements

Before adding the index:

Time: 4.163 min  
  - planning: 2.044 ms  
  - execution: 4.163 min  
    - I/O read: 12.013 min  
    - I/O write: 878.926 ms  
  
Shared buffers:  
  - hits: 64 (~512.00 KiB) from the buffer pool  
  - reads: 973281 (~7.40 GiB) from the OS file cache, including disk I/O  
  - dirtied: 20068 (~156.80 MiB)  
  - writes: 19766 (~154.40 MiB)

Postgres.ai explain: https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/15635/commands/54084

After adding the index:

Time: 2.496 ms  
  - planning: 2.097 ms  
  - execution: 0.399 ms  
    - I/O read: 0.340 ms  
    - I/O write: 0.000 ms  
  
Shared buffers:  
  - hits: 1 (~8.00 KiB) 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/54088

MR acceptance checklist

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