Use miniproj as a lighter alternative to proj.

As discussed in the Kanboard story, we could also replace the much more heavyweight proj by the lighter miniproj developed at Helmholtz GEOMAR. The main benefit would be fewer dependencies, especially built using C instead of Rust, and thereby hopefully better performance. The main downsides I see is that miniproj does not transform geometry but only points and that it does not directly transform between arbitrary CRS but only via WGS84 as an intermediate representation so there is much more manual work.