Show better type suggestions for generic stdlib types (PEP 585)

```py
a = set()
```
```
error: Need type annotation for "a" (hint: "a: Set[] = ...") [var-annotated]
```

If python-version >= 3.9 then it would be better to show something like
```
error: Cannot infer type parameter for "a" (hint: "a = set[]") [var-annotated]
```

Start by locating the diagnostic generation for the var-annotated error and the handling of the configured Python version. Compare the current Set hint with the requested PEP 585 form for Python 3.9 and newer; done when the suggestion uses the requested generic syntax only for those versions and existing behavior remains unchanged otherwise.

Written by the indexing model from the issue text.