Bump MailKit from 4.7.1.1 to 4.8.0 in /Backend.Tests

Bumps [MailKit](https://github.com/jstedfast/MailKit) from 4.7.1.1 to 4.8.0.
- [Changelog](https://github.com/jstedfast/MailKit/blob/master/ReleaseNotes.md)
- [Commits](jstedfast/MailKit@4.7.1.1...4.8.0)

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

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

Backend/BackendFramework.csproj

@@ -16,7 +16,7 @@
     <PackageReference Include="Microsoft.AspNetCore.Authentication.JwtBearer" Version="8.0.3" />
     <PackageReference Include="Microsoft.IdentityModel.Tokens" Version="7.5.1" />
     <PackageReference Include="System.IdentityModel.Tokens.Jwt" Version="7.5.1" />
-    <PackageReference Include="MailKit" Version="4.7.1.1" />
+    <PackageReference Include="MailKit" Version="4.8.0" />
     <PackageReference Include="MongoDB.Driver" Version="2.28.0" />
     <PackageReference Include="Swashbuckle.AspNetCore" Version="6.7.3" />
     <PackageReference Include="Xabe.FFmpeg" Version="5.2.6"/>