Getting rid of deprecated dependencies

[danlwarren]

This came in from CRAN:

ENMTools depends on (depends, imports or suggests) raster and one or
more of the retiring packages rgdal, rgeos or maptools
(https://r-spatial.org/r/2022/04/12/evolution.html). Since raster
3.6.3, all use of external FOSS library functionality has been
transferred to terra, making the retiring packages very likely
redundant. It would help greatly if you could remove dependencies on the
retiring packages as soon as possible.

[danlwarren]

We're going to have to make some decisions here about how much we want people's code to be backwards-compatible. We can write a bunch of checks that will convert raster to spatraster format on the fly, or we can just insist that people use spatraster from the get-go. I'm actually inclined to do the former for the next version, as I worry about people getting screwed over by updating the package halfway through a project. It's more work on our part, though.

[danlwarren]

Okay, after a solid week of Russell and myself working on this, I think it's done-ish. Good lord that was a lot of effort.