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/UserRep.cs

@@ -1,5 +1,7 @@
-using System.ComponentModel.DataAnnotations;
-using System.ComponentModel.DataAnnotations.Schema;
+using System.ComponentModel.DataAnnotations.Schema;
+#if NETFRAMEWORK
+using System;
+#endif
 
 namespace DigitalData.UserManager.Domain.Entities
 {
@@ -25,15 +27,31 @@ namespace DigitalData.UserManager.Domain.Entities
         public DateTime? ValidTo { get; set; }
 
         [ForeignKey("UserId")]
-        public virtual User? User { get; set; }
+        public virtual User
+#if NET6_0_OR_GREATER
+            ?
+#endif
+            User { get; set; }
 
         [ForeignKey("RepGroupId")]
-        public virtual Group? RepGroup { get; set; }
+        public virtual Group
+#if NET6_0_OR_GREATER
+            ?
+#endif
+            RepGroup { get; set; }
 
         [ForeignKey("GroupId")]
-        public virtual Group? Group { get; set; }
+        public virtual Group
+#if NET6_0_OR_GREATER
+            ?
+#endif
+            Group { get; set; }
 
         [ForeignKey("RepUserId")]
-        public virtual User? RepUser { get; set; }
+        public virtual User
+#if NET6_0_OR_GREATER
+            ?
+#endif
+            RepUser { get; set; }
     }
 }