Bump MailKit from 4.2.0 to 4.3.0 in /Backend.Tests

Bumps [MailKit](https://github.com/jstedfast/MailKit) from 4.2.0 to 4.3.0.
- [Changelog](https://github.com/jstedfast/MailKit/blob/master/ReleaseNotes.md)
- [Commits](jstedfast/MailKit@4.2.0...4.3.0)

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

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

Diff for: Backend/BackendFramework.csproj

@@ -18,7 +18,7 @@
     <PackageReference Include="System.IdentityModel.Tokens.Jwt" Version="6.33.0" />
     <PackageReference Include="Swashbuckle.AspNetCore" Version="6.5.0" />
     <PackageReference Include="MongoDB.Driver" Version="2.22.0" />
-    <PackageReference Include="MailKit" Version="4.2.0" />
+    <PackageReference Include="MailKit" Version="4.3.0" />
     <PackageReference Include="Xabe.FFmpeg" Version="5.2.6"/>
 
     <!-- SIL Maintained Dependencies. -->