rename `_SuppressWinFormsTrimError` to `_SuppressWinFormsTrimWarning`

### Is your feature request related to a problem? Please describe.

When a WinFoms project is using `true` but not `<_SuppressWinFormsTrimError>true` there will be an error:

>Microsoft.NET.Sdk\targets\Microsoft.NET.RuntimeIdentifierInference.targets(255,5): Error NETSDK1175: Windows Forms is not supported or recommended with trimming enabled.

This **error** is misleading.

### Describe the solution you'd like

As long as `_SuppressWinFormsTrimError` is added, the error is gone, so this is not an **error** in my point of view, it's more like an **warning**.

So it's better to rename `_SuppressWinFormsTrimError` to `_SuppressWinFormsTrimWarning`.

When `` is used but `_SuppressWinFormsTrimWarning` is not used, the AOT will run (this is a change), but with warning messages:
> Windows Forms is not recommended with trimming enabled.

Just say "not recommended" instead of "not supported or recommended".

When `_SuppressWinFormsTrimWarning` is used, it works as what it is now.

### Additional context

Most people don't know `_SuppressWinFormsTrimError`, it's not documented. Showing an error too early will prevent users from investigating into the other AOT options.