Illegal sized bounds: only suggest mutability change if needed

In a scenario like

```
struct Type;

pub trait Trait {
    fn function(&mut self)
    where
        Self: Sized;
}

impl Trait for Type {
    fn function(&mut self) {}
}

fn main() {
    (&mut Type as &mut dyn Trait).function();
}
```

the problem is Sized, not the mutability of self. Thus don't emit the
"you need &T instead of &mut T" note, or the other way around, as all
it does is just invert the mutability of whatever was supplied.

Fixes #103622.

compiler/rustc_hir_typeck/src/method/mod.rs

@@ -210,7 +210,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     ProbeScope::TraitsInScope,
                 ) {
                     Ok(ref new_pick) if pick.differs_from(new_pick) => {
-                        needs_mut = true;
+                        needs_mut = new_pick.self_ty.ref_mutability() != self_ty.ref_mutability();
                     }
                     _ => {}
                 }