Inappropriate error message for ambiguous record updates

When a record update is ambiguous under `DuplicateRecordFields` (i.e. there are multiple fields in scope with the same name as the field being updated) the error message says it requires a type signature. However support for disambiguating updates by type is being removed from future GHC versions (this starts in GHC 9.2 which discourages such updates with a warning).

For example:
```hs
{-# LANGUAGE DuplicateRecordFields #-}
data S = MkS { foo :: Int }
data T = MkT { foo :: Int }
t = (MkT 0) { foo = 2 }
```
gives rise to:
```
AmbiguousUpdate3.hs:4:5: error:
• Record update is ambiguous, and requires a type signature
• In the expression: (MkT 0) {foo = 2}
In an equation for ‘t’: t = (MkT 0) {foo = 2}
|
4 | t = (MkT 0) { foo = 2 }
| ^^^^^^^^^^^^^^^^^^^
```
However, adding a type signature gives:
```
AmbiguousUpdate3.hs:4:15: warning: [-Wambiguous-fields]
The record update (MkT 0) {foo = 2} with type T is ambiguous.
This will not be supported by -XDuplicateRecordFields in future releases of GHC.
|
4 | t = (MkT 0) { foo = 2 } :: T
| ^^^^^^^
```
I think we should change the message in the first case to remove "requires a type signature". But perhaps it could include some additional explanation about what "record update is ambiguous" means, and how to fix it?