Reword `?Trait` bound diagnostic

[compiler-errors]

I don't want to block this PR on surface level things like phrasing, so feel free to ignore this and I'll submit this as a follow-up PR. I've never liked how this specific diagnostic is formulated, it's super verbose.

Now with this being a hard error this sentence doesn't quite make sense anymore. I'd rather see a message that's straight to the point like `?` can only be applied to `Sized` (similar to `~const` can only be applied to `#[const_trait]` traits or `async` bound modifier only allowed on `Fn`/`FnMut`/`FnOnce` traits) and defer any further explanations to diagnostic note/help messages if at all (cc error codes).

If we wanted to feel fancy we could provide a structured suggestion for removing the entire bound.

Originally posted by @fmease in #135841 (comment)