Geo: Deprecate legacy Prometheus metrics

Due to migrations to SSF, some Geo Prometheus metrics, such as geo_wikis_*, are already no longer used. Others will soon no longer be used. Dedicated team has some Geo Grafana dashboards which need to be updated to use the new metric names.

• !135528 (merged) In the description of old Geo metrics in https://docs.gitlab.com/ee/administration/monitoring/prometheus/gitlab_metrics.html#sidekiq-metrics, say "This metric is deprecated in favor of X"
• In the Deprecations section of a release post, we should advertise these metric name changes - Deprecation issue