rust: upgrade to Rust 1.74.1

This is the next upgrade to the Rust toolchain, from 1.73.0 to 1.74.1
(i.e. the latest) [1].

See the upgrade policy [2] and the comments on the first upgrade in
commit 3ed03f4d ("rust: upgrade to Rust 1.68.2").

# Unstable features

No unstable features (that we use) were stabilized.

Therefore, the only unstable features allowed to be used outside the
`kernel` crate are still `new_uninit,offset_of`, though other code to
be upstreamed may increase the list (e.g. `offset_of` was added recently).

Please see [3] for details.

# Other improvements

Rust 1.74.0 allows to use `#[repr(Rust)]` explicitly [4], which can be
useful to be explicit about particular cases that would normally use
e.g. the C representation, such as silencing lints like the upcoming
additions we requested [5] to the `no_mangle_with_rust_abi` Clippy lint
(which in turn triggered the `#[repr(Rust)]` addition).

Rust 1.74.0 includes a fix for one of the false negative cases we reported
in Clippy's `disallow...