rust-toolchain: Bump toolchain version to 1.75.0

which, compared to the previous 1.70.0, has the following impacts on
`zellij`:

1. [cross-crate inlining][8]

This should increase application performance, as functions can now be
inlined across crates.

2. [`async fn` in traits][9]

This would allow us to drop the `async_trait` dependency, but it is
currently still required by 3 other dependencies.

Build time in debug mode (on my own PC) is cut down from 256s to 189s
(for a clean build). Build time in release mode is cut down from 473s to
391s (for a clean build). Binary sizes only change minimally (825 MB ->
807 MB in debug, 29 MB -> 30 MB in release).

[8]: rust-lang/rust#116505
[9]: rust-lang/rust#115822

rust-toolchain.toml

@@ -1,5 +1,5 @@
 # This file is updated by `update-toolchain.sh`
 [toolchain]
-channel = "1.70.0"
+channel = "1.75.0"
 components = ["rustfmt", "clippy", "rust-analysis"]
 targets = ["wasm32-wasi", "x86_64-unknown-linux-musl"]