Merge patch series "rust: bump minimum Rust and `bindgen` versions"

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 was released with `bindgen` 0.71.1, which it also still
uses to this day [4].

Debian Trixie's release happened on 2025-08-09 [5], 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 versions, i.e.

  - Rust: 1.78.0 -> 1.85.0
  - bindgen: 0.65.1 -> 0.71.1

There are a few main parts to the series, in this order:

  - A few cleanups that can be performed before the bumps.
  - The Rust bump (and its cleanups).
  - The `bindgen` bump (and its cleanups).
  - Documentation updates.
  - The `cfi_encoding` patch, added here, which needs the bump.
  - The per-version flags support and a Clippy cleanup on top.

Link: https://lwn.net/Articles/1050174/
Link: https://www.debian.org/releases/trixie/release-notes/whats-new.en.html#desktops-and-well-known-packages
Link: https://packages.debian.org/trixie/rustc
Link: https://packages.debian.org/trixie/bindgen
Link: https://www.debian.org/releases/trixie/
Link: https://patch.msgid.link/20260405235309.418950-1-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>