Webhook: fix exception with stream reader

[retailcoder]

Category: Microsoft.AspNetCore.Server.IIS.Core.IISHttpServer
EventId: 2
SpanId: d1d77d61055155f3
TraceId: c9a27a7446dd45e72696d2d8d1cf4913
ParentId: 0000000000000000
RequestId: 40000ae8-0000-f000-b63f-84710c7967bb
RequestPath: /webhook/github

Connection ID "17293822569908013799", Request ID "40000ae8-0000-f000-b63f-84710c7967bb": An unhandled exception was thrown by the application.

Exception: 
System.InvalidOperationException: Synchronous operations are disallowed. Call ReadAsync or set AllowSynchronousIO to true instead.
   at Microsoft.AspNetCore.Server.IIS.Core.HttpRequestStream.Read(Byte[] buffer, Int32 offset, Int32 count)
   at System.IO.StreamReader.ReadBuffer()
   at System.IO.StreamReader.ReadToEnd()
   at rubberduckvba.Server.WebhookAuthenticationHandler.<HandleAuthenticateAsync>b__2_0() in D:\a\rubberduckvba-website\rubberduckvba-website\rubberduckvba.Server\GitHubAuthenticationHandler.cs:line 78