Update metadata entry

Add Enablement/Memory to metadata

doc/administration/operations/sidekiq_memory_killer.md

 ---
-stage: none
-group: unassigned
+stage: Enablement
+group: Memory
 info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#designated-technical-writers
 ---