Bump Microsoft.Graph from 5.14.0 to 5.15.0 in /src (#2237)

Bumps [Microsoft.Graph](https://github.com/microsoftgraph/msgraph-sdk-dotnet) from 5.14.0 to 5.15.0.
- [Release notes](https://github.com/microsoftgraph/msgraph-sdk-dotnet/releases)
- [Changelog](https://github.com/microsoftgraph/msgraph-sdk-dotnet/blob/dev/CHANGELOG.md)
- [Commits](microsoftgraph/msgraph-sdk-dotnet@5.14.0...5.15.0)

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

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

src/AzureIoTHub.Portal.Server/AzureIoTHub.Portal.Server.csproj

@@ -60,7 +60,7 @@
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
     </PackageReference>
     <PackageReference Include="Microsoft.Extensions.Http.Polly" Version="7.0.7" />
-    <PackageReference Include="Microsoft.Graph" Version="5.14.0" />
+    <PackageReference Include="Microsoft.Graph" Version="5.15.0" />
     <PackageReference Include="Microsoft.OpenApi" Version="1.6.5" />
     <PackageReference Include="Microsoft.VisualStudio.Azure.Containers.Tools.Targets" Version="1.18.1" />
     <PackageReference Include="Microsoft.Identity.Web" Version="2.12.4" />