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/Group.cs

@@ -8,7 +8,11 @@ namespace DigitalData.UserManager.Domain.Entities
     public class Group : BaseEntity
     {
         [StringLength(50)]
-        public string? Name { get; set; }
+        public string
+#if NET6_0_OR_GREATER
+            ?
+#endif
+        Name { get; set; }
 
         [Required]
         [DefaultValue(false)]
@@ -24,12 +28,17 @@ namespace DigitalData.UserManager.Domain.Entities
         public bool Active { get; set; }
 
         [StringLength(200)]
-        public string? Comment { get; set; }
+        public string
+#if NET6_0_OR_GREATER
+            ?
+#endif
+            Comment
+        { get; set; }
 
         // TODO: this column should be assigned by triggers. despite this it is not null and this is problem for creation. talk with others
         [Required]
         [Column("ECM_FK_ID")]
         [DefaultValue(-1)]
-        public int EcmFkId { get; init; } = -1;
+        public int EcmFkId { get; set; } = -1;
     }
 }