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.

先檢查 samples/plus、docs/pagination.md 和 samples/appengine/main.py，issue 指出這些位置包含 Google+ 參照。在 samples 和文件中搜尋已移除 API 的參照，然後刪除或更新過時的參照；當已識別的過時 API 參照不再存在時，即表示完成。

由索引模型根據 Issue 內容生成。