Move to Microsoft.Identity.Web

[SeanKilleen]

Warnings in our latest build:

'AzureADAuthenticationBuilderExtensions.AddAzureAD(AuthenticationBuilder, Action)' is obsolete: 'This is obsolete and will be removed in a future version. Use AddMicrosoftWebApiAuthentication from Microsoft.Identity.Web instead. See https://aka.ms/ms-identity-web.'

'AzureADDefaults' is obsolete: 'This is obsolete and will be removed in a future version. Use Microsoft.Identity.Web instead. See https://aka.ms/ms-identity-web.'