Update project for multi-targeting and nullable types

Updated `DigitalData.UserManager.Domain.csproj` to support .NET 4.6.2, 7.0, 8.0, and 9.0. Adjusted implicit usings and nullable reference types settings based on the target framework, disabling nullable types for `net462` and enabling them for others.

Refactored entity classes (`BaseEntity`, `ClientUser`, `Group`, `GroupOfUser`, `Module`, `ModuleOfUser`, `User`, and `UserRep`) to conditionally include nullable reference types. Added `?` operator for string properties in .NET 7.0 and above, enhancing code safety and reducing null reference exceptions.

DigitalData.UserManager.Domain/Entities/UserRep.cs

@@ -28,28 +28,28 @@ namespace DigitalData.UserManager.Domain.Entities
 
         [ForeignKey("UserId")]
         public virtual User
-#if NET6_0_OR_GREATER
+#if NET7_0_OR_GREATER
             ?
 #endif
             User { get; set; }
 
         [ForeignKey("RepGroupId")]
         public virtual Group
-#if NET6_0_OR_GREATER
+#if NET7_0_OR_GREATER
             ?
 #endif
             RepGroup { get; set; }
 
         [ForeignKey("GroupId")]
         public virtual Group
-#if NET6_0_OR_GREATER
+#if NET7_0_OR_GREATER
             ?
 #endif
             Group { get; set; }
 
         [ForeignKey("RepUserId")]
         public virtual User
-#if NET6_0_OR_GREATER
+#if NET7_0_OR_GREATER
             ?
 #endif
             RepUser { get; set; }