Adds last_dormant_member_review_at to namespace settings

What does this MR do and why?

As part of [BE] Remove dormant members (#461340) we are going to allow customers to have their dormant members removed from a namespace hierarchy.

To handle the removal, we are going to use a limited capacity worker that can check for any remove dormant members on a daily basis (being handled in !157930 (merged)).

In order to do that, in this MR we are adding a last_dormant_member_review_at timestamp column to the NamespaceSettings table.

As recommended in !153100 (comment 1912378314), this migration also adds an index.

Refs #461340

Migration details

Migrating up:

bin/rails db:migrate
main: == [advisory_lock_connection] object_id: 128460, pg_backend_pid: 18416
main: == 20240701102528 AddLastDormantMemberReviewAtToNamespaceSettings: migrating ==
main: -- add_column(:namespace_settings, :last_dormant_member_review_at, :datetime_with_timezone)
main:    -> 0.0023s
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- view_exists?(:postgres_partitions)
main:    -> 0.0171s
main: -- index_exists?(:namespace_settings, [:last_dormant_member_review_at, :namespace_id], {:where=>"remove_dormant_members IS true", :name=>"idx_namespace_settings_on_last_dormant_member_review_at_id", :algorithm=>:concurrently})
main:    -> 0.0027s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0007s
main: -- add_index(:namespace_settings, [:last_dormant_member_review_at, :namespace_id], {:where=>"remove_dormant_members IS true", :name=>"idx_namespace_settings_on_last_dormant_member_review_at_id", :algorithm=>:concurrently})
main:    -> 0.0022s
main: -- execute("RESET statement_timeout")
main:    -> 0.0003s
main: == 20240701102528 AddLastDormantMemberReviewAtToNamespaceSettings: migrated (0.0396s) 

Migrating down:

 bin/rails db:migrate:down:main VERSION=20240701102528
main: == [advisory_lock_connection] object_id: 128100, pg_backend_pid: 14432
main: == 20240701102528 AddLastDormantMemberReviewAtToNamespaceSettings: reverting ==
main: -- remove_column(:namespace_settings, :last_dormant_member_review_at)
main:    -> 0.0032s
main: == 20240701102528 AddLastDormantMemberReviewAtToNamespaceSettings: reverted (0.0070s) 

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