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/User.cs

@@ -26,7 +26,11 @@ namespace DigitalData.UserManager.Domain.Entities
         [Required]
         [Column("USERNAME")]
         [StringLength(50)]
-        public string Username { get; set; }
+        public
+#if NET7_0_OR_GREATER
+            required
+#endif
+            string Username { get; set; }
 
         [Column("SHORTNAME")]
         [StringLength(30)]