Document migration from single database to decomposed for Streaming Replication setup

Document how self-managed instances using Streaming Replication can be use replication to migrate to multiple databases.

Instances with HA setup should be able to use this to reduce the downtime required - rough list of steps is at #429492 (comment 1895052629)

Note: The current instructions at https://docs.gitlab.com/ee/administration/postgresql/multiple_databases.html work for HA setup already but it likely requires significant downtime for large installations

Resources

• https://docs.gitlab.com/ee/administration/postgresql/replication_and_failover.html#near-zero-downtime-upgrade-of-postgresql-in-a-patroni-cluster
• &7791 (closed)
• https://docs.gitlab.com/ee/administration/postgresql/replication_and_failover.html#upgrading-postgresql-major-version-in-a-patroni-cluster
• https://docs.gitlab.com/omnibus/settings/database.html#upgrade-packaged-postgresql-server
• https://docs.gitlab.com/ee/update/zero_downtime.html
• https://docs.gitlab.com/ee/administration/postgresql/moving.html