Bump MimeKit from 2.5.2 to 2.6.0

Bumps [MimeKit](https://github.com/jstedfast/MimeKit) from 2.5.2 to 2.6.0.
- [Release notes](https://github.com/jstedfast/MimeKit/releases)
- [Changelog](https://github.com/jstedfast/MimeKit/blob/master/ReleaseNotes.md)
- [Commits](jstedfast/MimeKit@2.5.2...2.6.0)

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

Rnwood.Smtp4dev/Rnwood.Smtp4dev.csproj

@@ -26,7 +26,7 @@
 		<PackageReference Include="Microsoft.AspNetCore.StaticFiles" Version="2.2.0" />
 		<PackageReference Include="Microsoft.EntityFrameworkCore.InMemory" Version="3.1.3" />
 		<PackageReference Include="Microsoft.EntityFrameworkCore.Sqlite" Version="3.1.3" />
-		<PackageReference Include="MimeKit" Version="2.5.2" />
+		<PackageReference Include="MimeKit" Version="2.6.0" />
 		<PackageReference Include="Rnwood.SmtpServer" Version="3.1.0-ci0534" />
 		<PackageReference Include="Portable.BouncyCastle" Version="1.8.6.7" />
 		<PackageReference Include="System.Data.SQLite" Version="1.0.112" />