Create new index for removing artifacts with unknown locked status

What does this MR do and why?

This MR adds a new index for use by our Ci::JobArtifacts::UpdateUnknownLockedStatusService

The rationale and index design is laid out in our tracking issue.

Output of Migrations

From the db:check-migrations job

$ scripts/db_tasks db:migrate
== 20220510192117 IndexExpirableUnknownArtifactsForRemoval: migrating =========
-- index_exists?("ci_job_artifacts", [:expire_at, :job_id], {:name=>"tmp_index_ci_job_artifacts_on_expire_at_where_locked_unknown", :where=>"locked = 2 AND expire_at IS NOT NULL", :algorithm=>:concurrently})
   -> 0.0138s
-- add_index_options("ci_job_artifacts", [:expire_at, :job_id], {:name=>"tmp_index_ci_job_artifacts_on_expire_at_where_locked_unknown", :where=>"locked = 2 AND expire_at IS NOT NULL", :algorithm=>:concurrently})
   -> 0.0005s
== 20220510192117 IndexExpirableUnknownArtifactsForRemoval: migrated (0.0663s) 

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.