Remove support for monoandroid libraries

[sbomer]

Pre-.net6 binding libraries are likely to break with the new trimmable type map in .NET 11. This issue tracks removing support for these libraries.

Per our discussion @jonathanpeppers @simonrozsival the first step will be to add a warning when referencing old binding libraries (I think this means libraries with a monoandroid TFM).

Once that has had some time to bake, we can start removing custom steps and other logic that was there just for compatibility with such binding assemblies. We could also consider taking a breaking change to make these TFMs incompatible, similar to NuGet/Home#11338.

I'm attempting to add the warning in NuGet/NuGet.Client#7229.