Rebuild postgres-01 replica

Like discussed in https://gitlab.com/gitlab-com/infrastructure/issues/4893#note_99388982, let's rebuild the postgres replica (no full machine reprovisioning required, just getting a fresh basebackup and start off from there).

The intent is also to start over and see if the replication lag issue persists.

• Resetup replica
• Remove silences https://alerts.gprd.gitlab.net/#/silences
• Update in #production, ping @jurbanc
• Update runbooks with procedure