Upgrade to .NET Core 3.0

[Anduin2017]

This pull request upgrades this project from .NET Framework to .NET Core 3.0

[mmacy]

Hi @Anduin2017 and @EdiWang,

Please hold off on merging this PR until I have a chance to update the article on docs.microsoft.com that references it:

Azure AD B2C: Use the Azure AD Graph API

Once I have the update staged and ready to go, I'll let you know. We can then sync the merging of both so that the article isn't broken (that is, out of sync from the sample it references).

Cc: @jmprieur @jennyf19

[derisen]

Closing as we are archiving this repo. Please refer to instead: Azure AD B2C user account management with .NET Core and Microsoft Graph