Add .NET Framework 4.6.2 support and improve nullability

Updated `DigitalData.UserManager.Domain.csproj` to target .NET Framework 4.6.2 alongside .NET 7.0, 8.0, and 9.0. Corrected `PackageTags` from "user maanger" to "user manager" and added property groups for implicit usings and language versions.

Introduced nullable reference types in entity classes (`BaseEntity`, `ClientUser`, `Group`, `User`, etc.) for enhanced type safety. Updated properties in the `User` and `UserRep` classes to use the new nullable syntax, ensuring consistency across the codebase.

These changes improve compatibility with newer C# features and maintain support for multiple frameworks.

DigitalData.UserManager.Domain/Entities/GroupOfUser.cs

@@ -15,12 +15,24 @@ namespace DigitalData.UserManager.Domain.Entities
         public int GroupId { get; set; }
 
         [StringLength(200)]
-        public string? Comment { get; set; }
+        public string
+#if NET6_0_OR_GREATER
+            ?
+#endif
+            Comment { get; set; }
 
         [ForeignKey("UserId")]
-        public virtual User? User { get; set; }
+        public virtual User
+#if NET6_0_OR_GREATER
+            ?
+#endif
+            User { get; set; }
 
         [ForeignKey("GroupId")]
-        public virtual Group? Group { get; set; }
+        public virtual Group
+#if NET6_0_OR_GREATER
+            ?
+#endif
+            Group { get; set; }
     }
-}
+}