Bump MailKit from 2.8.0 to 2.9.0

Bumps [MailKit](https://github.com/jstedfast/MailKit) from 2.8.0 to 2.9.0.
- [Release notes](https://github.com/jstedfast/MailKit/releases)
- [Changelog](https://github.com/jstedfast/MailKit/blob/master/ReleaseNotes.md)
- [Commits](jstedfast/MailKit@2.8.0...2.9.0)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Rnwood.Smtp4dev.Tests/Rnwood.Smtp4dev.Tests.csproj

@@ -10,7 +10,7 @@
   </ItemGroup>
 
   <ItemGroup>
-    <PackageReference Include="MailKit" Version="2.8.0" />
+    <PackageReference Include="MailKit" Version="2.9.0" />
     <PackageReference Include="MedallionShell" Version="1.6.1" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.7.1" />
     <PackageReference Include="Selenium.Support" Version="3.141.0" />

Rnwood.Smtp4dev/Rnwood.Smtp4dev.csproj

@@ -21,7 +21,7 @@
 	<ItemGroup>
 		<PackageReference Include="CommandLiners.MonoOptions" Version="1.0.36" />
 		<PackageReference Include="HtmlAgilityPack" Version="1.11.24" />
-		<PackageReference Include="MailKit" Version="2.8.0" />
+		<PackageReference Include="MailKit" Version="2.9.0" />
 		<PackageReference Include="Microsoft.AspNet.OData" Version="7.4.1" />
 		<PackageReference Include="Microsoft.AspNetCore.Hosting.WindowsServices" Version="3.1.8" />
 		<PackageReference Include="Microsoft.AspNetCore.SignalR" Version="1.1.0" />