csharp_payment example outdated

[jlidbeck]

The csharp_payment project uses Square package version 22.0.0. The latest version is 42.0.2 and the API is completely different--the class structure and function names have changed and the Builder pattern has been removed, as documented here: https://developer.squareup.com/docs/sdks/dotnet/migration

Per the documentation:

SDK versions 40.1.0 and earlier continue to function as expected, but you should plan to update your integration as soon as possible to ensure continued support.

At the moment it is hard to find examples on the web of the new API. Code assist tools like Copilot also don't seem to know about the change, and generate incorrect old-style code, so this example would be useful to update.