Reassign E0406 to be thrown when undeclared associated type appears in function signature.

[nikhilshagri]

Currently, when an undeclared associated type is used in a function signature within a trait body, like:

trait Foo {
    type Bar;
    //error: Baz is used but not declared
    fn return_bool(&self, &Self::Bar, &Self::Baz) -> bool;
}

it throws E0220, which isn't really helpful in this situation. Also, I found E0406 to be the more appropriate error here. This error is thrown for some feature which hasn't been implemented yet( and probably won't be), so it can be repurposed. See #34230 for more.
We could modify the error message for E0220 to include the above erroneous code, but I think it would be better to throw E0406 instead of E0220 in this situation.

/cc @GuillaumeGomez

[GuillaumeGomez]

I'll try to take a look.

[eddyb]

E0406 is from some leftover code that shouldn't have existed in the first place.
E0220 is the correct error to use, message being:

error: associated type `Baz` not found for `Self``

The only problem I see is E0220's description needing to be updated. Also see #34342 (comment).