This is part of (or related to) MDLSITE-7418

In core we have a few uses of the "master/slave" terminology that better gets moved to something less controversial.

A quick scan has shown, for example, all the moodle_database related stuff with primary databases and read-only replicas.

No matter that we cannot change some SQLs (because they still continue using the master/slave alternatives in some databases... some of our own functions/configuration settings/comments... still are using master/slave. They are a good candidate to become deprecated and replace them by primary/replica alternative.

Note that, apart from that (moodle_database) case, there may be other candidates to apply for the same switch. This issue should fix as many of them as technically possible. We shouldn't be modifying 3rd part libraries in this issue.

Ciao