does not work as shown - please update for webforms 2020

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the following simply does not work. You always get a null! This needs updating for WebForms on ASP.Net 4.7.2!
var configurationValue = ConfigurationManager.AppSettings["ConfigurationSettingName"];

---
#### Document Details

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* ID: 1974914b-d7fa-8a3e-e7c0-3d80eb5d5079
* Version Independent ID: e24f4a05-bf1b-331e-b234-1c904ac95346
* Content: [App configuration](https://docs.microsoft.com/en-us/dotnet/architecture/blazor-for-web-forms-developers/config)
* Content Source: [docs/architecture/blazor-for-web-forms-developers/config.md](https://github.com/dotnet/docs/blob/master/docs/architecture/blazor-for-web-forms-developers/config.md)
* Product: **dotnet-architecture**
* Technology: **blazor**
* GitHub Login: @csharpfritz
* Microsoft Alias: **jefritz**