MailKit nuget package upgrade 4.14.1 -> 4.15.1

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: stidsborg

Samples/Cleipnir.Sample.Presentation/Cleipnir.Sample.Presentation.csproj

@@ -12,7 +12,7 @@
     </ItemGroup>
 
     <ItemGroup>
-        <PackageReference Include="MailKit" Version="4.14.1" />
+        <PackageReference Include="MailKit" Version="4.15.1" />
         <PackageReference Include="Serilog.Sinks.Console" Version="6.1.1" />
     </ItemGroup>
 </Project>