Build against libsoup3 by default

This follows the [recommendations from upstream](https://discourse.gnome.org/t/please-build-against-libsoup-3-by-default/10190/3). Note that this changes the option name from `soup3` to `soup2`.

Testing Done:
Compiled against libsoup3 and libsoup2.

Reviewed at https://reviews.imfreedom.org/r/2028/