Fixes #7706

We previously had a very specific backwards compatibility handler for
handling renamed fields. We always knew that wouldn't scale but I wanted
to wait for a real case. Well, #7706 is a real case, so here we are.

This commit makes our backwards compatibility handler more general
purpose, and makes a special-case handler for renamed fields built on
top of this same general purpose system. The new system lets us do a lot
more with regards to backwards compatibility.

To start, this addresses #7706 by allowing us to handle a removed single
enum value of a still-existing field.

It was unused and doesn't match our diagnostic API.

Rather than storing a list of errors we now store a list of
"diagnostics." Each diagnostic has a richer set of structured
information, including a message, a key, the location where it occurred.

This lets us show more detailed messages, more human friendly messages, and
also let's us filter by key or location. We don't take advantage of
all of this capability in this initial commit, but we do use every field
for something.