fix: ensured that allowed user name characters are supported in backoffice users as well (#14810) (#14825)

Co-authored-by: Arkadiusz Biel <bielu@bielu.com.pl>

src/Umbraco.Web.BackOffice/Security/ConfigureBackOfficeIdentityOptions.cs

@@ -38,6 +38,8 @@ public sealed class ConfigureBackOfficeIdentityOptions : IConfigureOptions<BackO
         options.SignIn.RequireConfirmedPhoneNumber = false; // not implemented
 
         options.User.RequireUniqueEmail = true;
+        // Support validation of users names using Down-Level Logon Name format
+        options.User.AllowedUserNameCharacters = _securitySettings.AllowedUserNameCharacters;
 
         options.ClaimsIdentity.UserIdClaimType = ClaimTypes.NameIdentifier;
         options.ClaimsIdentity.UserNameClaimType = ClaimTypes.Name;