Upgraded to Mastodon 4.2.5 to patch security bug.

Couple hiccups along the way because Ruby is, as it always has been and always will be, total trash.

Upgrade instructions leave out a crucial step from a previous point release where you'll get and error that you can install rbenv 3.2.3

`git -C /home/mastodon/.rbenv/plugins/ruby-build pull`