Refactor entity properties for nullability and requirements

Removed nullable indicators from string properties in
BaseEntity.cs, ClientUser.cs, Group.cs, and User.cs.
The Username property in User.cs is now marked as
required for .NET 7 or greater, improving data integrity
and reducing null reference risks.

DigitalData.UserManager.Domain/Entities/Group.cs

@@ -32,8 +32,7 @@ namespace DigitalData.UserManager.Domain.Entities
 #if NET7_0_OR_GREATER
             ?
 #endif
-            Comment
-        { get; set; }
+            Comment { get; set; }
 
         // TODO: this column should be assigned by triggers. despite this it is not null and this is problem for creation. talk with others
         [Required]