Do not propose unstable features on autocomplete for stable toolchain

[SomeoneToIgnore]

In the project with stable toolchain used, Some(22).ex<|> proposes expect_none method which is unstable currently.

I would expect RA not to propose unstable APIs when a stable toolchain is used.

[hecatia-elegua]

Could we put this behind some kind of setting?
Sometimes I search in completions to find a specific functionality and it happnes to be nightly-only. If I don't care about going nightly then, I do so. If I do care, I would want this setting.

Besides that, today we can easily discern feature-gated completions if they have an example: