Replace repmgr with Patroni

The Ongres folks recommend that we replace repmgr with Patroni here due to potential split-brain issues with repmgr: gitlab-com/migration#770 (comment 97123506)

That's unfortunate since we're shipping repmgr and supporting customers with this solution. Would upgrading to repmgr v4 improve the situation or should we switch outright?

/cc: @joshlambert, @ahachete, @ibaum, @marin