Until now, Rails only droped compatibility with older
rubies on new majors, but I propose to change this policy
because it causes us to either keep compatibility with long
EOLed rubies or to bump the Rails major more often, and to
drop multiple Ruby versions at once when we bump the major.
In my opinion it's a bad alignments of incentives. And we'd
be much better to just drop support in new minors whenever they
go EOL (so 3 years).
Also Ruby being an upstream dependency, it's not even
a semver violation AFAICT.
Since Rails 7.2 isn't planned before a few months, we
can already drop Ruby 3.0 as it will be EOL in March.
In Ruby 3.1 those gems were dropped from the stdlib, so they need to be
explicitly installed. Mail should be doing this for us, but since it
cares about Ruby < 2.6, and those gems can't be installed there, they
can't add them to the gemspec without dropping support to old rubies.
Since we don't care about Ruby < 2.7, we can just require them in all
frameworks that use mail.
