Rollup merge of #158468 - obi1kenobi:pg/default-body-stability, r=GuillaumeGomez

Include default-stability info in rustdoc JSON.

Add a `default_unstable` field on associated constants, associated types, and functions. The field is populated only when those items appear inside a trait, only when there's a default present, and when that default is not stable as designated by `#[rustc_default_body_unstable]`. In such a case, the field contains the name of the feature required to use the unstable default.

The purpose of this info is to allow `cargo-semver-checks` to lint the standard library for accidental breakage of stable APIs. Removing a stable default is an example of such breakage, while removing an _unstable_ default is not.

The field is boxed to minimize the size impact on its enclosing type, since for regular crates it will always be `None`.

I also updated `jsondoclint` to assert that it's an error to have a populated `default_unstable` when there's no function body, no default const value, or no default associated type. In the process, I noticed that `jsondoclint` and `jsondocck` are both on edition 2021 — I plan to upgrade them to 2024 in separate PRs.

r? @GuillaumeGomez

**AI disclosure:** This PR is the product of a combination of manual work and AI tools. I secured approval in advance from the designated reviewer. I stand behind the quality of the code I'm submitting, and I vouch it's as good or better compared to if I had written every line by my own hand.