Draft: main/rust: upgrade to 1.73.0

https://doc.rust-lang.org/nightly/releases.html#version-1730-2023-10-05

Trying to pick up where we left in !52887 (closed) but not up for adopting this aport myself.

I'm open to suggestions in this MR, of course, but also feel free to open your own MR to work on this, I don't mind.