Update to webkit2gtk-4.1

webkit2gtk-4.0 has been deprecated (see https://discourse.gnome.org/t/webkit2gtk-4-1-added-to-gnome-sdk-webkit2gtk-4-0-deprecated/7053).

Arch is already shipping only 4.1, so it'd be nice if 4.1 was used, so NetworkManager-openconnect can be built without any further adjustments.

This also implicitly requires libsoup3, as opposed to libsoup2 as with webkit2gtk-4.0. Due to this some small changes are required to auth-dialog.

It'd be nice if this could be included in the next release (see also #55 (closed)) if possible, since the webkit2gtk-4.0 requirement has been introduced before the user agent changing feature that I would like to use.

If my commit messages don't match your convention or you require some changes or similar, I'll be happy to make adjustments.