Make Tizen 6 map to NetStandard 2.1

[WonyoungChoi]

As a follow-up to #7762, a mapping for Tizen should be added to support NetStandard 2.1.

Tizen 6 (API Version 6, Platform Version 5.5) will be released in this year with the .NetCore 3.0 runtime.

In summary of Tizen TFM,

TFM	Supported Standard	Supported Tizen Platform	Mapping
Tizen3	.NET Standard 1.6	Tizen 4.0 preview	Explicitly mapping in NuGet.Client
Tizen4	.NET Standard 2.0	Tizen 4.0	Explicitly mapping in NuGet.Client
Tizen5	.NET Standard 2.0	Tizen 5.0	Implicit use
Tizen6	.NET Standard 2.1	Tizen 5.5	Explicitly mapping in NuGet.Client

[nkolev92]

Thanks for creating this @WonyoungChoi.

We will triage and get back to you accordingly.

[nkolev92]

Hey @WonyoungChoi, what's your time frame for this?
We will add the xamarin mappings later this year, so we're thinking 5.1 (Visual Studio 16.1), aligned with those changes.

[WonyoungChoi]

@nkolev92 The next Tizen release is scheduled for the end of October this year. In that release the NETCore 3.0 runtime (official version or preview version) will be available.

[nkolev92]

Thanks,
5.1 is the right timeframe then.