Avoid requiring `-fno-ignore-interface-pragmas` on Categorifier-using targets

With GHC 8.10, there is a new `dynflagsPlugin`. We should use that to set the flags we require for categorification. We should also 1. warn when we had to enable it (in case it is being explicitly disabled by the user) and 2. make sure it's clear that the flag is still required pre-8.10.

(Extricated from https://kitty-hawk.atlassian.net/browse/SW-2423)