MC 1.21.4 means we have to move more data generation code into the
client source set. Given all this code movement, it probably makes sense
to put data generation in a separate source set instead.
1.21.4 also has split data generators for client and server, but neither
mod loader recommends this. This means we can/should merge DataProviders
and ClientDataProviders into a single class.
Data generators are no longer bundled with the jar, which does reduce
file size, but by a tiny amount (~70KiB).