Add (async) index on target_project_id for MR violations

What does this MR do and why?

Add index on target_project_id for MR violations. This is a follow-up to a performance discussion on MR !120636

• Add index to speed up lookup of merge_request_compliance_violations for export to CSV

• Refs: Create Index on violations table columns [targe... (#424390 - closed) • Sam Figueroa • 16.4

• Refs: Export report from compliance violation report (#356791 - closed) • Sam Figueroa • 16.4 • On track

Screenshots or screen recordings

$> rake db:migrate

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main: == [advisory_lock_connection] object_id: 226220, pg_backend_pid: 80090
main: == 20230906181457 AddIndexToViolationsOnTargetProjId: migrating ===============
main: -- index_exists?("merge_requests_compliance_violations", [:target_project_id, :id], {:name=>"i_compliance_violations_for_export", :algorithm=>:concurrently})
main:    -> 0.0037s
main: -- add_index_options("merge_requests_compliance_violations", [:target_project_id, :id], {:name=>"i_compliance_violations_for_export", :algorithm=>:concurrently})
main:    -> 0.0001s
main: == 20230906181457 AddIndexToViolationsOnTargetProjId: migrated (0.1939s) ======

main: == [advisory_lock_connection] object_id: 226220, pg_backend_pid: 80090 

ci: == [advisory_lock_connection] object_id: 226200, pg_backend_pid: 82139
ci: == 20230906181457 AddIndexToViolationsOnTargetProjId: migrating ===============
ci: -- index_exists?("merge_requests_compliance_violations", [:target_project_id, :id], {:name=>"i_compliance_violations_for_export", :algorithm=>:concurrently})
ci:    -> 0.0040s
ci: -- add_index_options("merge_requests_compliance_violations", [:target_project_id, :id], {:name=>"i_compliance_violations_for_export", :algorithm=>:concurrently})
ci:    -> 0.0003s
ci: == 20230906181457 AddIndexToViolationsOnTargetProjId: migrated (0.1921s) ======

ci: == [advisory_lock_connection] object_id: 226200, pg_backend_pid: 82139

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.