Bump MimeKit from 3.0.0 to 4.7.1 in /MailingNinja.Core

Bumps MimeKit from 3.0.0 to 4.7.1.

---
updated-dependencies:
- dependency-name: MimeKit
  dependency-type: direct:production
...

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

MailingNinja.Core/MailingNinja.Core.csproj

@@ -13,7 +13,7 @@
 		<PackageReference Include="MediatR.Extensions.Microsoft.DependencyInjection" Version="9.0.0" />
 		<PackageReference Include="MediatR" Version="9.0.0" />
 		<PackageReference Include="FluentValidation.AspNetCore" Version="10.3.4" />
-		<PackageReference Include="MimeKit" Version="3.0.0" />
+		<PackageReference Include="MimeKit" Version="4.7.1" />
   </ItemGroup>
 
   <ItemGroup>