Build(deps): Bump MimeKit from 4.5.0 to 4.6.0 (#137)

Bumps [MimeKit](https://github.com/jstedfast/MimeKit) from 4.5.0 to 4.6.0.
- [Changelog](https://github.com/jstedfast/MimeKit/blob/master/ReleaseNotes.md)
- [Commits](jstedfast/MimeKit@4.5.0...4.6.0)

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

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

TuviPgpLibImpl/TuviPgpLibImpl.csproj

@@ -19,7 +19,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="MimeKit" Version="4.5.0" />
+    <PackageReference Include="MimeKit" Version="4.6.0" />
   </ItemGroup>
 
   <ItemGroup>

TuviPgpLibTests/TuviPgpLibTests.csproj

@@ -22,7 +22,7 @@
 
   <ItemGroup>
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.9.0" />
-    <PackageReference Include="MimeKit" Version="4.5.0" />
+    <PackageReference Include="MimeKit" Version="4.6.0" />
     <PackageReference Include="Moq" Version="4.20.70" />
     <PackageReference Include="NUnit" Version="4.1.0" />
     <PackageReference Include="NUnit3TestAdapter" Version="4.5.0" />