[CLOUD-80] monitoring: add `perms_syncer_outdated_perms` panel

Created by: unknwon

Adds a Prometheus metric and corresponding Grafana panel for tracking number of entities with outdated permissions, currently the type of records is only "user".

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• Jira: https://sourcegraph.atlassian.net/browse/CLOUD-80
• Followup of https://github.com/sourcegraph/sourcegraph/pull/26081