Per discussion, there's no need to automatically break our ecosystem with an upper version constraint, since we already have a published model for breaking changes.

Remove the upper constraint from:
https://github.com/flutter/flutter/blob/master/packages/flutter_tools/templates/package/pubspec.yaml.tmpl
https://github.com/flutter/flutter/blob/master/packages/flutter_tools/templates/plugin/pubspec.yaml.tmpl