`prefer_equal_for_default_values` is removed from Dart 3.0.0

See https://github.com/dart-lang/sdk/blob/main/CHANGELOG.md#linter

This is still present in https://github.com/flutter/flutter/blob/0db47bdd07afedaf62de17b88a815833a593e21f/analysis_options.yaml#L156