rust-bindings: regenerate for gtk-rs 0.15

[lucab]

This updates our Rust bindings to version 0.15 of the gtk-rs stack, see https://gtk-rs.org/blog/2022/01/16/new-release.html.
Notably, we now require a Rust toolchain >= 1.56 and GLib/GObject/GIO >= 2.48.

Closes: #2647

[cgwalters]

That seems easier than I'd expected! Did you hit anything surprising or problematic?

[lucab]

Did you hit anything surprising or problematic?

Notable things I spotted while assembling this PR:

• NONE_CANCELLABLE is gone. This cascades quite a bit to consumers.
• ostree_repo_finder_avahi_start() is actually missing the usual bool return value. May be worth replacing with a _try_start() with a better signature.
• some misgenerated GString code on ostree_gpg_verify_result_describe*(), likely a gir-rs bug. Still need to look into it, just ignoring the two functions for now.

[lucab]

Ok, this one should be ready to merge.
Once in, we can do the semver bumps and release crates ostree-sys 0.11.0 and ostree 0.16.0.

[cgwalters]

One thing to keep in mind - cap-std is heading to 1.0 hopefully soon: bytecodealliance/cap-std#272

And that will require another round of semver bumps for us, but it will thankfully be the last from that side.

[lucab]

Ah yes, that one comes with a fairly large MSRV jump though.
On the gtk-rs there is also https://gtk-rs.org/blog/2022/10/18/new-release.html which just came out and also requires a toolchain >= 1.63.
I think we can do a semver bump here for this, and then bundle the new cap-std and gtk-rs together for another bump in a few months when the dust settles. How do you feel about this plan?

[cgwalters]

SGTM

[lucab]

Crates update for this is at #2749.

I've noted down the future plans for the next semver bump with both gtk-rs and cap-std at #2750.