5.3 Sunset

This is a tag prior to a series of updates I'll make on the codebase to make the transition to 5.6 easier, as well as a cutoff point for putting 5.3 in "maintenance mode".

The breadth of changes and deprecations is unexpectedly large when it comes to PCGEx, so the strategy I'm adopting is massaging the shared codebase to make the actual 5.6 easier, because there's just too much deprecation to keep track of; and I want to still be able to quickly back-port potential new new node from 5.6 to 5.5/5.4

There will be no more PCGEx udpates for 5.3 from that point onwards.