Claim ServiceDeskSetting custom_email

Mario Celirequested to merge
562081-claim-service_desk_settings.custom_email into master

What does this MR do and why?

Claiming uniqueness across cells for column service_desk_settings.custom_email

Changes were already made to the topology service gem to include the new types at gitlab-org/cells/topology-service!464 (merged) and the gitlab project uses the new version of the gem with !223411 (merged)

Related to #562081 (closed)

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