[2.x] Require 10.3 minimum and reduce flood related code

Problem/Motivation

From berdir:

That said, once you *require* 10.3 (as in, it's the min version you support) you should be able to delete large chunks of code and just rely on the new UserAuthenticationInterface::lookupAccount(). You don't need to duplicate the password checks in UserAuth, you don't need form validate trickery to get flood protection, because providing lookupAccount() will integrate with that nicely. And you can reduce your UserAuthenticationController to a few lines (or change it to a request subscriber) that maps email to name and then call the parent. That said, I didn't look through all the options you provide. You still need a bit of form alter to change the label and so on.

Proposed resolution

Remaining tasks

do it

User interface changes

n/a

API changes

n/a

Data model changes

n/a