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/User.cs

@@ -2,75 +2,101 @@
 using System.ComponentModel.DataAnnotations;
 using System.ComponentModel.DataAnnotations.Schema;
 
-namespace DigitalData.UserManager.Domain.Entities;
-
-[Table("TBDD_USER", Schema = "dbo")]
-public class User : BaseEntity
+namespace DigitalData.UserManager.Domain.Entities
 {
-    [Column("PRENAME")]
-    [StringLength(50)]
-    public string? Prename { get; set; }
+    [Table("TBDD_USER", Schema = "dbo")]
+    public class User : BaseEntity
+    {
+        [Column("PRENAME")]
+        [StringLength(50)]
+        public string
+#if NET6_0_OR_GREATER
+            ?
+#endif
+            Prename { get; set; }
 
-    [Column("NAME")]
-    [StringLength(50)]
-    public string? Name { get; set; }
+        [Column("NAME")]
+        [StringLength(50)]
+        public string
+#if NET6_0_OR_GREATER
+            ?
+#endif
+            Name { get; set; }
 
-    [Required]
-    [Column("USERNAME")]
-    [StringLength(50)]
-    public string Username { get; set; }
+        [Required]
+        [Column("USERNAME")]
+        [StringLength(50)]
+        public string Username { get; set; }
 
-    [Column("SHORTNAME")]
-    [StringLength(30)]
-    public string? Shortname { get; set; }
+        [Column("SHORTNAME")]
+        [StringLength(30)]
+        public string
+#if NET6_0_OR_GREATER
+            ?
+#endif
+            Shortname
+        { get; set; }
 
-    [Column("EMAIL")]
-    [StringLength(100)]
-    public string? Email { get; set; }
+        [Column("EMAIL")]
+        [StringLength(100)]
+        public string
+#if NET6_0_OR_GREATER
+            ?
+#endif
+            Email { get; set; }
 
-    [Required]
-    [Column("LANGUAGE")]
-    [StringLength(5)]
-    [DefaultValue("de-DE")]
-    public string Language { get; set; }
+        [Required]
+        [Column("LANGUAGE")]
+        [StringLength(5)]
+        [DefaultValue("de-DE")]
+        public string
+#if NET6_0_OR_GREATER
+            ?
+#endif
+            Language { get; set; }
 
-    [Column("COMMENT")]
-    [StringLength(500)]
-    public string? Comment { get; set; }
+        [Column("COMMENT")]
+        [StringLength(500)]
+        public string
+#if NET6_0_OR_GREATER
+            ?
+#endif
+            Comment { get; set; }
 
-    [Column("DELETED")]
-    public bool Deleted { get; set; }
+        [Column("DELETED")]
+        public bool Deleted { get; set; }
 
-    [Required]
-    [Column("DATE_FORMAT")]
-    [StringLength(10)]
-    [DefaultValue("dd.MM.yyyy")]
-    public string DateFormat { get; set; }
+        [Required]
+        [Column("DATE_FORMAT")]
+        [StringLength(10)]
+        [DefaultValue("dd.MM.yyyy")]
+        public string DateFormat { get; set; }
 
-    [Required]
-    [Column("ACTIVE")]
-    public bool Active { get; set; }
+        [Required]
+        [Column("ACTIVE")]
+        public bool Active { get; set; }
 
-    #region IGNORED COLUMNS
-    //[Required]
-    //[Column("GENERAL_VIEWER")]
-    //[StringLength(30)]
-    //[DefaultValue("NONE")]
-    //public string GeneralViewer { get; set; }
+        #region IGNORED COLUMNS
+        //[Required]
+        //[Column("GENERAL_VIEWER")]
+        //[StringLength(30)]
+        //[DefaultValue("NONE")]
+        //public string GeneralViewer { get; set; }
 
-    //[Required]
-    //[Column("WAN_ENVIRONMENT")]
-    //public bool WanEnvironment { get; set; }
+        //[Required]
+        //[Column("WAN_ENVIRONMENT")]
+        //public bool WanEnvironment { get; set; }
 
-    //[Required]
-    //[Column("USERID_FK_INT_ECM")]
-    //public int UseridFkIntEcm { get; set; }
+        //[Required]
+        //[Column("USERID_FK_INT_ECM")]
+        //public int UseridFkIntEcm { get; set; }
 
-    //[Column("DELETED_WHEN")]
-    //public DateTime? DeletedWhen { get; set; }
+        //[Column("DELETED_WHEN")]
+        //public DateTime? DeletedWhen { get; set; }
 
-    //[Column("DELETED_WHO")]
-    //[StringLength(50)]
-    //public string? DeletedWho { get; set; }
-    #endregion
+        //[Column("DELETED_WHO")]
+        //[StringLength(50)]
+        //public string? DeletedWho { get; set; }
+        #endregion
+    }
 }