[Announcement] Deprecate flash(ActionScript) client generator

Created by: ybelenko

December 31, 2020 is end of life for Adobe Flash Player. As soon as there is no usage evidence(open issues, pull requests) of flash generator I think we can deprecate it now.

It's a kinda sad for me personally because it was my first programming language and the first tech book I read (Essential ActionScript 3.0 published June 2007 by O'Reilly Media, Inc.).