Setup db indicator to gitlab_main_cell schema

As discussed on #429871 (comment 1664327227), we need to enable gitlab_main_cell schema configuration to be able to use all DB health checks when using the defer Sidekiq Workers feature for tables which the schema is gitlab_main_cell

To accomplish this, we will have to create change request issues to update the prometheus_alert_db_indicators_settings application setting on Staging and Production, similar to gitlab-com/gl-infra/production#16138 (closed) and gitlab-com/gl-infra/production#16191 (closed)

Also, create an MR to add the gitlab_main_cell schema to PrometheusAlertIndicator