In the past I've been more cautious, but this is the version supported
by Debian Stable, and it's more than 12 months old.
A decent number of dependencies are only tested against newer rustc
versions (see e.g. the recent aho-coarasick pin due to no MSRV on that
library). I've filed bugs in some cases, but upgrading rustc versions
more aggressively should make this problem less common.
Difftastic is generally conservative about MSRV, and will only
increase the version when there is a compelling reason (e.g. major
performance improvement, important bug fix in a dependendency).
This version increase will enable us to upgrade crossterm to 0.26, which
has better detection of terminal width on Windows.
I've also clarified MSRV details for other dependencies that cannot
currently be upgraded.