Adding support for cross-cluster associations to Rails 7

[todsacerdoti]

https://github.blog/2021-07-12-adding-support-cross-cluster-associations-rails-7/

[ksec]

>At GitHub, we have 30 databases configured in our Rails monolith—15 primaries and 15 replicas.

For some reason, ( possibly because of Planetscale ) I was under the impression Github was using Vitess.