rename _SuppressWinFormsTrimError to _SuppressWinFormsTrimWarning

[felixf4xu]

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

When a WinFoms project is using <PublishAot>true</PublishAot> but not <_SuppressWinFormsTrimError>true</_SuppressWinFormsTrimError> 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 <PublishAot> 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.

[marcpopMSFT]

@merriemcgaw since this is the winforms development experience.

[merriemcgaw]

Chatting with the team and will circle back.