Bump Microsoft.OpenApi from 1.6.17 to 1.6.21 in /src

Bumps [Microsoft.OpenApi](https://github.com/Microsoft/OpenAPI.NET) from 1.6.17 to 1.6.21.
- [Release notes](https://github.com/Microsoft/OpenAPI.NET/releases)
- [Commits](microsoft/OpenAPI.NET@1.6.17...1.6.21)

---
updated-dependencies:
- dependency-name: Microsoft.OpenApi
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

src/IoTHub.Portal.Server/IoTHub.Portal.Server.csproj

@@ -62,7 +62,7 @@
     </PackageReference>
     <PackageReference Include="Microsoft.Extensions.Http.Polly" Version="8.0.8" />
     <PackageReference Include="Microsoft.Graph" Version="5.56.0" />
-    <PackageReference Include="Microsoft.OpenApi" Version="1.6.17" />
+    <PackageReference Include="Microsoft.OpenApi" Version="1.6.21" />
     <PackageReference Include="Microsoft.VisualStudio.Azure.Containers.Tools.Targets" Version="1.21.0" />
     <PackageReference Include="Microsoft.Identity.Web.UI" Version="2.21.1" />
     <PackageReference Include="MudBlazor" Version="6.21.0" />