rust-bindings/cargo: bump to gtk-rs 0.15

[lucab]

Next iteration of the gtk-rs stack is out, see https://gtk-rs.org/blog/2022/01/16/new-release.html.
This effectively means bumping glib/gio/gobject to 0.15.

However, this comes with an hard requirement for a minimum Rust toolchain >= 1.56.
Holding this for now while we verify that such toolchain requirement is fine for the environments we target.
Also, this bumps the C native dependencies:

GLib/GObject/GIO: minimum 2.48, latest 2.72

[lucab]

This is going be a large diff change and also a semver-break for the library, with some cost for the whole stack of consumers.
We had a chat out-of-band and we'd like to relocate this repo to #2575 first.