–Upgrade to .NET 11

Elwig/Windows/MailWindow.xaml.cs

@@ -954,7 +954,7 @@ namespace Elwig.Windows {
                             body.Add(doc.AsEmailAttachment($"{name}.pdf"));
                         }
                         msg.Body = body;
-                        await client!.SendAsync(msg);
+                        await client!.SendAsync(new SmtpSendRequest(msg));
                         await Utils.AddSentMails([(
                             "email", m.MgNr, m.AdministrativeName,
                             m.EmailAddresses.OrderBy(a => a.Nr).Select(a => a.Address).ToArray(),