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

[pintusingh28]

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