Allow deprecated heapless::mpmc to fix no_std builds

[cart]

Objective

heapless just deprecated their mpmc module in a minor version, which broke our no_std builds (because we choose to break our break our builds on warnings).

rust-embedded/heapless#583
https://docs.rs/heapless/latest/heapless/mpmc/index.html

Solution

Short term, fix our builds by allowing this usage of a deprecated function. However it is worth investigating: #21819

[BenjaminBrienen]

You might have to allow allow_attribute

[BenjaminBrienen]

Weird unrelated change. I expect that rustfmt prevents this.

[cart]

I believe this is actually because this file wasnt rustfmt-ed for some reason.

[mockersf]

the no std job is not required by CI so its failures are not blocking, and the deprecation is used there to mark a real issue. I would prefer to have the investigation in #21819

from their doc:

The current implementation of mpmc is marked as deprecated due to not being truly lock-free

and

In that case you can safely ignore the warnings using #[expect(deprecated)] when new is called

So it's not actually deprecated, but used to mark that it doesn't work as expected in some cases