Bump MimeKit from 2.15.1 to 3.4.2

Bumps [MimeKit](https://github.com/jstedfast/MimeKit) from 2.15.1 to 3.4.2.
- [Release notes](https://github.com/jstedfast/MimeKit/releases)
- [Changelog](https://github.com/jstedfast/MimeKit/blob/master/ReleaseNotes.md)
- [Commits](jstedfast/MimeKit@2.15.1...3.4.2)

---
updated-dependencies:
- dependency-name: MimeKit
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

Author: dependabot[bot]

DNN Platform/Library/packages.config

@@ -13,7 +13,7 @@
   <package id="Microsoft.SourceLink.Common" version="1.1.1" targetFramework="net472" developmentDependency="true" />
   <package id="Microsoft.SourceLink.GitHub" version="1.1.1" targetFramework="net472" developmentDependency="true" />
   <package id="Microsoft.Web.Infrastructure" version="2.0.0" targetFramework="net472" />
-  <package id="MimeKit" version="2.15.1" targetFramework="net472" />
+  <package id="MimeKit" version="3.4.2" targetFramework="net472" />
   <package id="Newtonsoft.Json" version="13.0.1" targetFramework="net472" />
   <package id="PetaPoco.Compiled" version="6.0.524" targetFramework="net472" />
   <package id="Portable.BouncyCastle" version="1.9.0" targetFramework="net472" />