Improve DB performance of Issuable finders

What does this MR do and why?

This MR tries to improve performance of issuables fetching query. No user facing changes here.

Database migration

main: == 20220602130306 AddNamespaceTypeIndex: reverting ============================
main: -- transaction_open?()
main:    -> 0.0000s
main: -- indexes(:namespaces)
main:    -> 0.0682s
main: -- remove_index(:namespaces, {:algorithm=>:concurrently, :name=>"index_groups_on_parent_id_id"})
main:    -> 0.0066s
main: == 20220602130306 AddNamespaceTypeIndex: reverted (0.0928s) ===================

main: == 20220602130306 AddNamespaceTypeIndex: migrating ============================
main: -- transaction_open?()
main:    -> 0.0000s
main: -- index_exists?(:namespaces, [:parent_id, :id], {:where=>"type = 'Group'", :name=>"index_groups_on_parent_id_id", :algorithm=>:concurrently})
main:    -> 0.0599s
main: -- add_index(:namespaces, [:parent_id, :id], {:where=>"type = 'Group'", :name=>"index_groups_on_parent_id_id", :algorithm=>:concurrently})
main:    -> 0.0113s
main: == 20220602130306 AddNamespaceTypeIndex: migrated (0.0859s) ===================

Query plans

Query	Before	After	Performance stats
Issues count	87k hits 200ms	85k hits 170ms	+- the same
MRs count	113k hits 140ms	56k hits 100ms	40% faster, 50% less data reads
Issues list	113k hits 230ms	85k hits 230ms	20% less data reads
MRs list	109k hits 190ms	52k hit 120ms	60% faster, 50% less data reads

There is one more problem I found: primary key of project_authorizations seem to be corrupted or something. It's not picked up in query plan but if you remove it and recreate again - it will be picked up. I'm not sure if it's database-lab only problem. Also not sure on the best approach how to fix it. Fixing that will save us few thousands buffer reads.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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

Related to #361687 (closed)