lfs: move the 'supportedoutgoingversions' handling to changegroup.py
This handling already exists here for the narrow extension. We still need to
either figure out how to enable changegroup v3 without the extension, or figure
out how to let the server detect that the client doesn't have it loaded, and
emit a user friendly error[1]. I can't tell if D1944 is the appropriate vehicle
for the latter.
[1] https://www.mercurial-scm.org/pipermail/mercurial-devel/2018-January/109550.html
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