Remove `EquatableMixin` and mark `Equatable` class with `abstract mixin class`

As the latest version of dart has removed support for dart version `< 3.0.0`, we should consider to updated dart sdk constraints to `>=3.0.0 <4.0.0` and replace `EquatableMixin` with `Equatable` with marking it as `abstract mixin class`, so we can use `Equatable` class as Mixin or Base class whichever is requied.

**Solution:**
- Update Dart sdk constraints to `>=3.0.0 <4.0.0`
- Mark `Equatable` class as `abstract mixin class`
- Deprecate `EquatableMixin` and remove it in future versions.

reopens the issue #194