Geo: Allow GraphQL mutations for Geo in maintenance mode

What does this MR do and why?

This MR allows Geo mutations to resync and reverify individual and multiple registries if maintenance mode is enabled. These mutations can only be run by admins, and they are legitimate queries in some contexts, like Geo failover.

An example of a valid use case:

• Turn on Maintenance Mode
• Let Geo get to 100% synced and verified
• Something is failing to sync
• Resolve the underlying problem
• Trigger a resync (for example in Admin UI, which would use this GraphQL query)
• 100% synced and verified
• Failover
• Turn off Maintenance Mode

Besides, this MR updates the docs to include this ability when maintenance mode is enabled.

How to set up and validate locally

• Follow https://docs.gitlab.com/ee/administration/maintenance_mode/#enable-maintenance-mode

In a Geo secondary:

• Go to http://<GDK-HOST>/admin/geo/sites/2/replication/snippet_repositories (or any other replicable).
• Perform a resync or reverify actions by clicking one of the action buttons.
• Check if the registry is resynced or reverified correctly.

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.

Related to #411328 (closed)