[Issue] Confirm-ProxyServer does not work on Powershell 7

Describe the issue

The Confirm-ProxyServer shared function always fails on PowerShell 7. Run any script with -Verbose and observe:

Calling Confirm-ProxyServer
Proxy server configuration detected
Unable to check for proxy server configuration

On PowerShell 7, this line throws, so we always end up in the catch:

GetSystemWebProxy() is deprecated and there is a new way to do this: https://github.com/dotnet/runtime/issues/24756#issuecomment-519618435

[System.Net.Http.HttpClient]::DefaultProxy.GetProxy("foo")

However, that doesn't work in Windows PowerShell.

Not clear if we should fix this or just let it continue to work this way. In PS7 we will never try to use a proxy. This apparently didn't cause an issue for customers running CVE-2023-23397.ps1. I know we had some using PS7 when running against Exchange Online.

Start at the shared Confirm-ProxyServer function and reproduce the failure by running a script with -Verbose on PowerShell 7. Compare the Windows PowerShell behavior of GetSystemWebProxy() with [System.Net.Http.HttpClient]::DefaultProxy.GetProxy("foo"), then verify that proxy detection works appropriately in both PowerShell versions.