review requested on PR to remove old compat code

I asked on -users, and have turned up zero actual people who say they
are using pre-2.52.0, so "remove the compat code" is decided.

https://urldefense.com/v3/__https://github.com/bcpierce00/unison/pull/1174__;!!IBzWLUs!TJ5PcXohhu9OV18Usz0w5HgSIwTq37hmCeBhUDGU_ln_Ob_p9K1CGqmCQquTTMPlIMLEBsvqWTXJuiyEuA8rniwo$ 

I would appreciate review, about the choice of where exactly to draw the
line at old-code removal and the implications for next steps, as well as
the usual.

I am not in a rush to make a release, but would like to merge this in
say 4 days, absent review comments arguing otherwise.

After merge, and perhaps a few other minor changes, I envision calling
for testing as a more-or-less release candidate, letting things sit
for a month or so, and then releasing 2.53.9.