Retire VeggieSeasons (really this time) (#2724)

Removes VeggieSeaons 

## Pre-launch Checklist

- [x] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [x] I signed the [CLA].
- [x] I read the [Contributors Guide].
- [x] I have added sample code updates to the [changelog].
- [x] I updated/added relevant documentation (doc comments with `///`).

---------

Co-authored-by: Loïc Sharma <737941+loic-sharma@users.noreply.github.com>

veggieseasons/README.md

@@ -1,33 +1,3 @@
-# Veggie Seasons
+# Sample deprecated
 
-An iOS sample app that shows which fruits and vegetables are currently in season. It
-showcases Flutter's Cupertino package.
-
-**NOTE:** While Flutter supports many platforms, this application is designed
-specifically for iOS. It's not intended to be run on Android, web, or desktop.
-
-## Goals
-
-* Show how to build an interface that iOS users will feel right at home
-  with.
-* Show how Flutter's Cupertino widgets work together.
-
-## The important bits
-
-### `/screens/*`
-
-These are the screens presented in the app, roughly analogous to
-UIViewControllers. `HomeScreen` is the root, and the others are shown
-as the user navigates.
-
-## Questions/issues
-
-If you have a general question about any of the techniques you see in
-the sample, the best places to go are:
-
-* [The FlutterDev Google Group](https://groups.google.com/forum/#!forum/flutter-dev)
-* [The Flutter Gitter channel](https://gitter.im/flutter/flutter)
-* [StackOverflow](https://stackoverflow.com/questions/tagged/flutter)
-
-If you run into an issue with the sample itself, please file an issue
-in the [main Flutter repo](https://github.com/flutter/flutter/issues).
+The `veggieseasons` sample has been deprecated. It now lives in the [flutter/demos](https://github.com/flutter/demos) repository, where it will no longer be maintained.