Schedule async index build for ci_job_artifacts

Our purpose-built index for removal of expired records from
ci_job_artifacts does not exclude artifacts where expire_at is null.
This means job traces, which get unlocked but have no expiration, are
polluting an index that would otherwise be a clean ready-for-removal
queue and much smaller.

Changelog: performance

db/post_migrate/20220325000000_prepare_index_for_ci_job_artifacts_unlocked_with_expire_at.rb

+# frozen_string_literal: true
+
+class PrepareIndexForCiJobArtifactsUnlockedWithExpireAt < Gitlab::Database::Migration[1.0]
+  TABLE_NAME = 'ci_job_artifacts'
+  INDEX_NAME = 'index_ci_job_artifacts_on_expire_at_for_removal'
+  CONDITIONS = 'locked = 0 AND expire_at IS NOT NULL'
+
+  def up
+    prepare_async_index TABLE_NAME, [:expire_at], where: CONDITIONS, name: INDEX_NAME
+  end
+
+  def down
+    unprepare_async_index_by_name TABLE_NAME, INDEX_NAME
+  end
+end

db/schema_migrations/20220325000000

+fd738cf8a5642f96dc67a294dcf913d797b580a19f35d5ef76573c63bf2dd920
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