Incompatible with dart sdk 3.0

[Xyndra]

Error:

The current Dart SDK version is 3.0.0-229.0.dev.

Because simple_school depends on network_state any which doesn't support null safety, version solving failed.

The lower bound of "sdk: '>=2.2.2 <3.0.0'" must be 2.12.0 or higher to enable null safety.
For details, see https://dart.dev/null-safety