Add index to projects on marked_for_deletion_at field is null

What does this MR do and why?

Add index to projects table on marked_for_deletion_at column where marked_for_deletion_at is NULL. We have a the scope not_aimed_for_deletion, however, this is not optimised and uses a sequential scan. We will be using this scope at several places in the follow-up MR !75742 (merged) and therefore need to create an index to optimise the query.

Database

Query Plan

SELECT id FROM projects WHERE marked_for_deletion_at IS NULL limit 10000

  Limit  (cost=0.44..236.49 rows=10000 width=4) (actual time=0.106..5.779 rows=10000 loops=1)
   Buffers: shared hit=6390
   I/O Timings: read=0.000 write=0.000
   ->  Index Only Scan using index_projects_not_aimed_for_deletion on public.projects  (cost=0.44..524181.76 rows=22205709 width=4) (actual time=0.103..4.205 rows=10000 loops=1)
         Heap Fetches: 375
         Buffers: shared hit=6390
         I/O Timings: read=0.000 write=0.000

https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/7507/commands/26712

Migrations

Up

== 20211203091642 AddIndexToProjectsOnMarkedForDeletionAt: migrating ==========
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:projects, :id, {:where=>"marked_for_deletion_at IS NULL", :name=>"index_projects_not_aimed_for_deletion", :algorithm=>:concurrently})
   -> 0.0158s
-- execute("SET statement_timeout TO 0")
   -> 0.0005s
-- add_index(:projects, :id, {:where=>"marked_for_deletion_at IS NULL", :name=>"index_projects_not_aimed_for_deletion", :algorithm=>:concurrently})
   -> 0.0092s
-- execute("RESET statement_timeout")
   -> 0.0006s
== 20211203091642 AddIndexToProjectsOnMarkedForDeletionAt: migrated (0.0338s) =

https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/7507/commands/26711

Down

== 20211203091642 AddIndexToProjectsOnMarkedForDeletionAt: reverting ==========
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:projects, :id, {:name=>"index_projects_not_aimed_for_deletion", :algorithm=>:concurrently})
   -> 0.0161s
-- execute("SET statement_timeout TO 0")
   -> 0.0005s
-- remove_index(:projects, {:name=>"index_projects_not_aimed_for_deletion", :algorithm=>:concurrently, :column=>:id})
   -> 0.0185s
-- execute("RESET statement_timeout")
   -> 0.0008s
== 20211203091642 AddIndexToProjectsOnMarkedForDeletionAt: reverted (0.0447s) =

https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/7507/commands/26713

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Relates to #344013 (closed)