See flutter/packages#3993 (comment) for context.

Currently we aren't setting this in plugins. Questions to answer:

• Should we be? What are the impacts on clients (e.g., does it change the whole app's compat behaviors, in which case it would be quite dangerous)?
• Is there a benefit to setting it, for us and/or for clients?
• What should our policy be for rolling it? Should we require integration (and potentially manual) testing of the plugin in an example with that target SDK first?
• What do we want to enforce about in CI tooling? Presumably we shouldn't ever let it be higher than the highest targetSdkVersion of any example app it has that runs integration tests?