Adapted for latest `rand` APIs

[finalyards]

The version of rand used in the project has fallen behind, mainly due to p256 still relying on the earlier version.

I wanted to give it a try, and attached is a PR that:

• raises rand to 0.10.0-rc.0 (currently latest)
• cleans up some dependencies that seem to be not used
• fixes some conditional compilation, especially with regard to rand (security feature)
• occasional non-functional cleanup..

The changes to 0.9.x were extensive, covering not only the API but the whole rand ecosystem, as well.

I chose to push past the 0.9, since we already can see what the 0.10 API would look like. Happy to keep maintaining this, as a PR, until the 0.10 proper is released.

Some questions to authors in the comments below. I need guidance.

Discussed previously: https://github.com/embassy-rs/trouble/issues/423