Switch to stable Rust, remove the nightly feature

[ivmarkov]

On Dec 28, Rust 1.75 was released which finally allows using async traits with the stable compiler.

Therefore, this PR removes the nightly feature flag which used to protect all async traits defined in the rs-matter crate.

We no longer need a nightly compiler, yay!

There is an upcoming bigger, subsequent PR which - besides updating a few dependencies - will also introduce the embedded-nal-async crate into the codebase, which would - in turn - allow us to get rid of the hard-coded UDP stack implementations based on async-io and embassy-net, and therefore remove the async-io and embassy-net as dependencies.

• embassy-net does implement the embedded-nal-async traits directly
• The Rust STD socket layer is adapted to the embedded-nal-async traits by the std-embedded-nal-async crate, but we do not have to depend on it directly, of course. Users can pick and choose.

[CLAassistant]

All committers have signed the CLA.