Migrate to connection string ingestion

[baywet]

detailed error message
D:\github\microsoft-graph-devx-api\GraphWebApi\Startup.cs(43,17): warning CS0618: 'ApplicationInsightsServiceOptions.InstrumentationKey' is obsolete: 'InstrumentationKey based global inge
stion is being deprecated. Use ApplicationInsightsServiceOptions.ConnectionString. See microsoft/ApplicationInsights-dotnet#2560 for more details.'