Bump Rust to 1.75 (#3815)

**This PR also bumps wasmtime version from 0.38 to 2.0 as 0.38 does not
build with Rust 1.75**. I did not test the plugin runtime as (AFAIK) we
intend to deprecate it; also, wasmtime's most recent version is 16.0, so
it'd make sense to bump the version at some point anyways. I did not
bump the version to 16.0 straight away as that'd require code changes in
`plugin_runtime`.
Release Notes:

- N/A

crates/gpui/src/util.rs

@@ -1,6 +1,5 @@
 use smol::future::FutureExt;
 use std::{future::Future, time::Duration};
-pub use util::*;
 
 pub fn post_inc(value: &mut usize) -> usize {
     let prev = *value;