rust: bump Rust minimum supported version to 1.85.0 (Debian Trixie)

As proposed in the past in e.g. LPC 2025 and the Maintainers Summit [1],
we are going to follow Debian Stable's Rust versions as our minimum
supported version.

Debian Trixie was released with a Rust 1.85.0 toolchain [2], which it
still uses to this day [3] (i.e. no update to Rust 1.85.1).

Debian Trixie's release happened on 2025-08-09 [4], which means that a
fair amount of time has passed since its release for kernel developers
to upgrade.

Thus bump the minimum to the new version.

Then, in later commits, clean up most of the workarounds and other bits
that this upgrade of the minimum allows us.

pin-init was left as-is since the patches come from upstream. And the
vendored crates are unmodified, since we do not want to change those.

Note that the minimum LLVM major version for Rust 1.85.0 is LLVM 18 (the
Rust upstream binaries use LLVM 19.1.7), thus e.g. `RUSTC_LLVM_VERSION`
tests can also be updated, but there are no suitable ones to simplify.

Ubuntu 25.10 also has a recent enough Rust toolchain [5], and they also
provide versioned packages with a Rust 1.85.1 toolchain even back to
Ubuntu 22.04 LTS [6].

Link: https://lwn.net/Articles/1050174/ [1]
Link: https://www.debian.org/releases/trixie/release-notes/whats-new.en.html#desktops-and-well-known-packages [2]
Link: https://packages.debian.org/trixie/rustc [3]
Link: https://www.debian.org/releases/trixie/ [4]
Link: https://packages.ubuntu.com/search?suite=all&searchon=names&keywords=rustc [5]
Link: https://launchpad.net/ubuntu/+source/rustc-1.85 [6]
Acked-by: Tamir Duberstein <tamird@kernel.org>
Acked-by: Benno Lossin <lossin@kernel.org>
Acked-by: Gary Guo <gary@garyguo.net>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Acked-by: Alice Ryhl <aliceryhl@google.com>
Link: https://patch.msgid.link/20260405235309.418950-6-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

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