rust: bump `bindgen` minimum supported version to 0.71.1 (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 `bindgen` versions as our minimum
supported version.

Debian Trixie was released with `bindgen` 0.71.1, which it still uses
to this day [2].

Debian Trixie's release happened on 2025-08-09 [3], 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.

Ubuntu 25.10 also has a recent enough `bindgen` [4] (even the already
unsupported Ubuntu 25.04 had it), and they also provide versioned packages
with `bindgen` 0.71.1 back to Ubuntu 24.04 LTS [5].

Link: https://lwn.net/Articles/1050174/ [1]
Link: https://packages.debian.org/trixie/bindgen [2]
Link: https://www.debian.org/releases/trixie/ [3]
Link: https://packages.ubuntu.com/search?suite=all&searchon=names&keywords=bindgen [4]
Link: https://launchpad.net/ubuntu/+source/rust-bindgen-0.71 [5]
Acked-by: Tamir Duberstein <tamird@kernel.org>
Reviewed-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260405235309.418950-18-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>