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/BaseEntity.cs

@@ -1,6 +1,9 @@
 using System.ComponentModel;
 using System.ComponentModel.DataAnnotations;
 using System.ComponentModel.DataAnnotations.Schema;
+#if NETFRAMEWORK
+using System;
+#endif
 
 namespace DigitalData.UserManager.Domain.Entities
 {
@@ -13,11 +16,21 @@ namespace DigitalData.UserManager.Domain.Entities
 
         [StringLength(50)]
         [Column("ADDED_WHO")]
-        public string? AddedWho { get; set; }
+        public string
+#if NET6_0_OR_GREATER
+            ?
+#endif
+            AddedWho
+        { get; set; }
 
         [StringLength(50)]
         [Column("CHANGED_WHO")]
-        public string? ChangedWho { get; set; }
+        public string
+#if NET6_0_OR_GREATER
+            ?
+#endif
+            ChangedWho
+        { get; set; }
 
         //TODO: assign it to default value in create dto, not here!
         [Column("ADDED_WHEN", TypeName = "datetime")]