Add has_merge_request column to vulnerability_reads

What does this MR do and why?

This is a migration to add column to vulnerability_reads table.

Related to #371313 (closed)

Database review

rake db:migrate:up

main: == [advisory_lock_connection] object_id: 224160, pg_backend_pid: 65295
main: == 20230728193736 AddHasMergeRequestToVulnerabilityReads: migrating ===========
main: -- transaction_open?()
main:    -> 0.0000s
main: -- add_column(:vulnerability_reads, :has_merge_request, :boolean, {:default=>false, :if_not_exists=>true})
main:    -> 0.0764s
main: == 20230728193736 AddHasMergeRequestToVulnerabilityReads: migrated (0.0940s) ==

main: == [advisory_lock_connection] object_id: 224160, pg_backend_pid: 65295

rake db:migrate:down

main: == [advisory_lock_connection] object_id: 223900, pg_backend_pid: 66264
main: == 20230728193736 AddHasMergeRequestToVulnerabilityReads: reverting ===========
main: -- remove_column(:vulnerability_reads, :has_merge_request, {:if_exists=>true})
main:    -> 0.0802s
main: == 20230728193736 AddHasMergeRequestToVulnerabilityReads: reverted (0.0833s) ==

main: == [advisory_lock_connection] object_id: 223900, pg_backend_pid: 66264

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.