💚 Update Application Insights to use Connection String instead of Instrumentation Key

[jernejk]

cc: @wicksipedia @adamcogan

Microsoft changed the recommended way of configuring Application Insights from using Instrumentation Keys to Connection Strings. As per my conversation with @wicksipedia, we should update this.

Tasks

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1. Update the code that loads the connection string
2. Update environment variables from APPINSIGHTS_INSTRUMENTATIONKEY to APPINSIGHTS_CONNECTIONSTRING
3. Update GitHub environment variables

[jernejk]

Done but needs #651 to be deployable.