Implemented host filtering to restrict connections to `localhost`
outside of development environments. Updated
`Program.cs` to include `AddHostFiltering` and `UseHostFiltering`.

Removed `AllowedHosts` from `appsettings.json` as it is now
hardcoded. Updated `README.md` to clarify this behavior.

Co-authored-by: Keegan Caruso <keegancaruso@microsoft.com>