Audit samples and docs for references to old APIs (like G+).

Issue #2646 was closed due to inactivity with a note saying "re-open if you think this still requires attention". It still requires attention, but no permission to reopen to issue. (Which seems like a rude combination.)

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Was looking through stuff and discovered that there are still references to Google+:

https://github.com/googleapis/google-api-python-client/tree/main/samples/plus

https://github.com/googleapis/google-api-python-client/blob/8b109032b71ca0b95b106af3ce470b7193d73148/docs/pagination.md?plain=1#L7

https://github.com/googleapis/google-api-python-client/blob/8b109032b71ca0b95b106af3ce470b7193d73148/samples/appengine/main.py#L73

Seeing references to removed APIs immediately makes me question the correctness of everything in this repository.