Support new webkitgtk-6.0 API version

Paraphrasing Michael Catanzaro:

WebKitGTK 2.39.1 has just been released, removing the webkit2gtk-5.0 API
version, and adding a new webkitgtk-6.0 API version instead.
webkitgtk-6.0 API version instead (note the 2 is gone). There will be
frequent soname bumps going forward, but most likely gnome-initial-setup
will be unaffected by any API changes because it uses very little of the
WebKitGTK API.

Use Meson 0.60.0's support for passing multiple names to dependency(),
using the first one found.  Bump the minimum Meson version accordingly.

https://mesonbuild.com/Reference-manual_functions.html#dependency
(cherry picked from commit bfb0e6a9)