Cleanup after DropInvalidSecurityFindings migration

What does this MR do and why?

This MR performs cleanup after DropInvalidSecurityFindings background migration by

• calling finalize_background_migration
• removing tmp_index_uuid_is_null index on security_findings table

Related to #346100 (closed)

Database review

db:migrate and db:rollback

➜ bin/rails db:migrate
== 20211123161906 CleanupAfterDropInvalidSecurityFindings: migrating ==========
-- transaction_open?()
   -> 0.0000s
-- indexes(:security_findings)
   -> 0.0033s
-- execute("SET statement_timeout TO 0")
   -> 0.0005s
-- remove_index(:security_findings, {:algorithm=>:concurrently, :name=>"tmp_index_uuid_is_null"})
   -> 0.0026s
-- execute("RESET statement_timeout")
   -> 0.0007s
== 20211123161906 CleanupAfterDropInvalidSecurityFindings: migrated (0.0275s) =

➜ bin/rails db:rollback
== 20211123161906 CleanupAfterDropInvalidSecurityFindings: reverting ==========
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:security_findings, :id, {:where=>"uuid IS NULL", :name=>"tmp_index_uuid_is_null", :algorithm=>:concurrently})
   -> 0.0029s
-- execute("SET statement_timeout TO 0")
   -> 0.0006s
-- add_index(:security_findings, :id, {:where=>"uuid IS NULL", :name=>"tmp_index_uuid_is_null", :algorithm=>:concurrently})
   -> 0.0047s
-- execute("RESET statement_timeout")
   -> 0.0006s
== 20211123161906 CleanupAfterDropInvalidSecurityFindings: reverted (0.0113s) =

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.