Update README to match current supported platforms

[tjaalton]

README.md has not been updated with current platform support, and legacy ones should be dropped.

Keeping it updated helps downstreams with having their package descriptions match the current state.

[AdamCetnerowski]

Thanks for catching the oversight. We'll correct it shortly.

[hbent]

This is still an issue. Gen8 and 9 platforms have apparently been dropped per 19e2dd0 but the github landing page is still claiming that they're supported.

[JablonskiMateusz]

README has been updated in 8327cdb