Mark OS Specific APIs

[HurricanKai]

In Net5.0 and onwards the Platform Compatability Analyzer will be enabled by default.
We should mark OS Specific APIs using SupportedOSPlatform and UnsupportedOSPlatform.

[HurricanKai]

@Perksey do upstreams mark such OS specific APIs somehow?

[Perksey]

Unfortunately not, this will have to be functionality defined in generator.json somehow.

[HurricanKai]

That's unfortunate.
We should in the future also look into targeting only the platforms that we want to target, which automatically hides stuff like iOS packages from windows targeting people. (net5.0 onwards)

[HurricanKai]

In 3.0 we want to exclusively target net6.0 so assigning this to that milestone.
Adding this to both build tools and silk touch, might be able to make that a silk touch feature.
Also, ideally we can get the attributes in for 2.0