Deprecate SDKs prior to Dart 3.2 / Flutter 3.16 #5911

When Flutter 3.35 was released in Aug 2025, this was announced:

https://blog.flutter.dev/whats-new-in-flutter-3-35-c58ef72e3766#:~:text=In%20our%20next%20stable%20release%2C%20Flutter%20SDKs%20before%203.16%20will%20be%20deprecated.

In our next stable release, Flutter SDKs before 3.16 will be deprecated.

At the end of Dev 2025 we started warning that these SDKs would soon be unsupported, and with the upcoming release we will stop supporting them.