deps: Bump MimeKit from 4.14.0 to 4.15.0

---
updated-dependencies:
- dependency-name: MimeKit
  dependency-version: 4.15.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

benchmarks/Zetian.Compare.Benchmarks/Zetian.Compare.Benchmarks.csproj

@@ -16,7 +16,7 @@
 
     <ItemGroup>
         <PackageReference Include="MailKit" Version="4.14.1" />
-        <PackageReference Include="MimeKit" Version="4.14.0" />
+        <PackageReference Include="MimeKit" Version="4.15.0" />
         <PackageReference Include="SmtpServer" Version="11.1.0" />
         <PackageReference Include="BenchmarkDotNet" Version="0.15.8" />
     </ItemGroup>