bindings: rust: bump MSRV to 1.60

bindings: rust: bump MSRV to 1.60

Specifying MSRV (minimum supported rust version) was introduced with
Rust 1.56. So old versions of libraries do not have this restriction
spelled out in their Cargo.toml.

This means that even if we claimed a MSRV of 1.56 until now, that did
not mean that things were actually buildable with 1.56 practically
(without manually researching compatible versions and pinning them
down).

`bindgen` started listing a MSRV from v0.61 (requiring Rust 1.57) [1].
So that may seem like an obvious choice. But if one attempts to build
it with such an old version one will realize that `log` only started
requesting a MSRV with 0.4.19 (requesting Rust 1.60) [2]. Hence, we
would either need to manually restrict log to an old, but compatible
release (which would unnecessarily restrict everyone on newer Rust
versions) or just also bump our MSRV to 1.60.

Rust 1.60 was released on 2022-04-07 [3] and seems like an acceptable
choice if core components such as `log` started their MSRV tracking with
that version. If someone is determined wanting to use this with an older
version of Rust, thats still possible. If one has done the necessary
manual research and pinned their versions down, one can use
--ignore-rust-version (or a < 1.56 version of Rust that does not check
the MSRV yet).

Thanks to Manos Pitsidianakis <manos.pitsidianakis@linaro.org> for
helping me out when I successfully confused myself somewhere.

[1] https://github.com/rust-lang/rust-bindgen/blob/v0.61.0/bindgen/Cargo.toml
[2] https://github.com/rust-lang/log/blob/0.4.19/Cargo.toml
[3] https://blog.rust-lang.org/2022/04/07/Rust-1.60.0.html

Signed-off-by: Erik Schilling <erik.schilling@linaro.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>