[Dart] Increase min SDK to last minor Dart 1 release

[parlough]

On the road to explicitly requiring Dart 2.

[alexander-doroshko]

Thanks,
I'm in contact with @jwren on how to increase it and when

[jwren]

Thanks @parlough and @alexander-doroshko. At the start of the year we will have the freedom and flexibility to remove versions before Dart SDK 3.0.0 (Flutter SDK version 3.10.0, mapping here: https://docs.flutter.dev/release/archive).

Change in the Flutter Plugin for IntelliJ nudging users who are on such an old version: flutter/flutter-intellij@451654b

@itsjustkevin

[parlough]

Thank you both! Now that the Flutter IntelliJ notification and policy have been out for a bit, I went ahead and opened #942 instead of this PR. It takes a bit of a more conservative approach for now while still getting many of the benefits.