Bump MailKit from 4.7.1.1 to 4.16.0

---
updated-dependencies:
- dependency-name: MailKit
  dependency-version: 4.16.0
  dependency-type: direct:production
...

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

Author: dependabot[bot]

src/SmtpServer.Tests/SmtpServer.Tests.csproj

@@ -5,7 +5,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="MailKit" Version="4.7.1.1" />
+    <PackageReference Include="MailKit" Version="4.16.0" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.11.1" />
     <PackageReference Include="xunit" Version="2.9.1" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.8.2">