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.
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