Things we should drop in v15. The list is probably not complete. This includes everything deprecated in v13.x.

• The @run_async decorator
• Filters.{private, group}
• utils.{promise, webhookhandler}
• clean argument of Updater.start_{polling, webhook}
• Think about removing Defaults from tg.Bot completey and moving them to tg.ext.ExtBot. We may have to think about how to handle signatures with DefaultVaule, then, i.e. if _insert_defaults should stay in tg.Bot or should be moved to tg.ext.ExtBot.