Bump Microsoft.AspNetCore.WebSockets in /src/WebSocketApp

Bumps [Microsoft.AspNetCore.WebSockets](https://github.com/aspnet/AspNetCore) from 2.1.1 to 2.1.7.
- [Release notes](https://github.com/aspnet/AspNetCore/releases)
- [Changelog](https://github.com/dotnet/aspnetcore/blob/main/docs/ReleasePlanning.md)
- [Commits](dotnet/aspnetcore@2.1.1...v2.1.7)

---
updated-dependencies:
- dependency-name: Microsoft.AspNetCore.WebSockets
  dependency-type: direct:production
...

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Directory.Packages.props

@@ -11,7 +11,7 @@
     <PackageVersion Include="Microsoft.AspNetCore.App" Version="2.1.34" />
     <PackageVersion Include="Giraffe" Version="3.1.0" />
     <PackageVersion Include="Giraffe.Razor" Version="1.3.0" />
-    <PackageVersion Include="Microsoft.AspNetCore.WebSockets" Version="2.1.1" />
+    <PackageVersion Include="Microsoft.AspNetCore.WebSockets" Version="2.1.7" />
     <PackageVersion Include="TaskBuilder.fs" Version="2.1.0" />
   </ItemGroup>
 </Project>