Add .NET 6.0 TFM to packages

[akoeplinger]

Fixes #4237

See dotnet/designs#222 (comment) for context.

@mconnew I verified these packages restore using the net6.0 assets in the HelloAndroid sample from https://github.com/dotnet/maui-samples and at least a basic call to a SOAP webservice works.

[mconnew]

@ericstj, can you comment on whether we need to add net6.0 to the Ref projects too? This work is to make the latest Xamarin platform which is based on .NET 6.0 (as opposed to mono) to use the WCF runtime assets from our packages instead of the previous Xamarin in-box mono based implementation.

[akoeplinger]

Yes we do need them in the Ref projects too since the packages contain ref/MonoAndroid10/_._ right now as well, so we need the same explicit net6.0 asset as for lib.

[akoeplinger]

@mconnew this should be good to go now, Eric gave his thumbs up :)

[ericstj]

Yes, to state explicitly: please make parallel changes to TFM additions in ref/src. You need both here.