Remove problematic language and use updated terminology for replicas

[JesKingDev]

The language used to refer to read-only replicas should be updated to match the official Redis docs and to address problematic language.

I propose using the term replica for all variables, comments, etc. - ex. here

Other terms that should be changed can be found in this inclusive language guide More on the "why" behind this

[colinmollenhour]

No objections to your proposal, Jessica. Please provide a PR that doesn't introduce any compatibility issues. -Thanks