Add deprecation warning for monoandroid TFMs

[sbomer]

NuGet Product(s) Affected

dotnet.exe, NuGet.exe

Current Behavior

The monoandroid TFMs are compatible with net6.0-android and above.

Desired Behavior

Referencing a monoandroid TFM library from net11.0-android and later should produce a warning. We are planning to introduce breaking changes that will prevent monoandroid libraries from functioning correctly in net11.0-android and above.

Additional Context

Android issue at dotnet/android#11028.
NuGet/NuGet.Client#7229 is adding the warning for monoandroid references.
NuGet/docs.microsoft.com-nuget#3542 is adding a doc.

[dotnet-policy-service]

For changes in NuGet functionality (such as NuGet Restore changes), refer to the .NET SDK breaking change guidelines