Enhance framework compatibility and code readability

Added preprocessor directives for .NET framework compatibility.
Modified `using` directives to be framework-specific.
Improved code formatting for better readability.
Introduced obsolete attributes for deprecated methods,
recommending `MediatR` as an alternative.
Added XML documentation for clarity and maintainability.

DigitalData.Core.Abstraction.Application/EntityExtensions.cs

@@ -1,4 +1,5 @@
-namespace DigitalData.Core.Abstraction.Application;
+#if NET
+namespace DigitalData.Core.Abstraction.Application;
 
 /// <summary>
 /// Provides extension methods for retrieving the value of an 'Id' property from objects.
@@ -91,4 +92,5 @@ public static class EntityExtensions
 #pragma warning restore CS8601
         return id is not null;
     }
-}
+}
+#endif