Drop Ruby 2.4 support

[tenderlove]

Ruby core has dropped all support for Ruby 2.4. Should we drop it as well? I know this is kind of cheating, but we have failing builds on Ruby 2.4 and I don't think it's worth our time to fix them if Ruby 2.4 isn't supported anymore.

cc @sodabrew

[sodabrew]

My recent hiatus from using Ruby as a daily-driver aside, I think back to how much time I spent making sure that old versions of Ruby continued to work in several gems...

If we did make Ruby 2.5+ required, I would want to do a major version bump. And actually, it's been on my mind for a looong time that the next major should be 1.0. There's nothing that substantially changed in this API for many releases and it would allow Rails to relax/remove its version requirement in the mysql2 adapter to prevent runtime version errors.

The really salient problem I think is also compelling to figure out a policy for is what to do about people who've been posting that they want to backport newer-MySQL and newer-MariaDB support to mysql2 0.3.x so that they can continue to use older versions of Rails with newer versions of MySQL 8.

[sodabrew]

Closing this out as CI is cleaned up again and thank you for the shepherding 0.5.4 release!

At some point older Ruby versions can be dropped, but so far we can punt that to another day ⚽