Bump Dart SDK version to 2.12.0 for the example

[nilsreichardt]

Required to run flutter analyze with a Flutter version +3.10. Otherwise, you get this error:

$ flutter analyze
Resolving dependencies... 
  collection 1.17.2 (1.18.0 available)
  material_color_utilities 0.5.0 (0.8.0 available)
  stack_trace 1.11.0 (1.11.1 available)
  stream_channel 2.1.1 (2.1.2 available)
  test_api 0.6.0 (0.6.1 available)
Got dependencies!
Resolving dependencies in ./example... 
The lower bound of "sdk: '>=2.1.0 <3.0.0'" must be 2.12.0'
or higher to enable null safety.

The current Dart SDK (3.1.0) only supports null safety.

For details, see https://dart.dev/null-safety