Clarify that if the Rust compiler releases which apply for a coding guideline are unknown, mark as "unknown"

[PLeVasseur]

Applies to:

• https://github.com/rustfoundation/safety-critical-rust-coding-guidelines/blob/main/.github/ISSUE_TEMPLATE/CODING-GUIDELINE.yml
• Any place else where we might give guidance, maybe CONTRIBUTING.md

[rcseacord]

We also have a rule that is basically going to say Use at least Rust Edition 2024 #252

The Rust 2024 Edition is stabilized and released with Rust version 1.85.0

So discussing versions before 1.85.0 is pointless, especially if we make this rule mandatory.

[PLeVasseur]

Not so clear cut, and we probably need an edition label.

Why? Many safety-qualified Rust compilers are still on the 2021 edition.

That'd matter for certain things which are added in new editions so they are classified appropriately.

[rcseacord]

@PLeVasseur we should consider #252 first. This draft mandatory guideline says that you can't create safe systems using anything older than 1.85.0
If we agree with this, then functionally safe code will require a newer version of a certified compiler.
The general rule here is that if you can't do it safely, you can't do it.