`cgi` module is deprecated

See [PEP 594](https://peps.python.org/pep-0594/#cgi). The `cgi` module is deprecated since version Python 3.11, will be removed in version Python 3.13.

Currently, a `DeprecationWarning` is emitted due to `appengine-python-standard` use of the `cgi` module.

```
.venv/lib/python3.12/site-packages/google/appengine/runtime/request_environment.py:29: DeprecationWarning: 'cgi' is deprecated and slated for removal in Python 3.13
import cgi
```

## Specifications

- Version: appengine-python-standard v1.1.6
- Platform: Linux 6.6.30-2 x86_64; Python 3.12.2

Start with the request_environment.py:29 location shown in the traceback and read the PEP 594 CGI deprecation details. Determine the supported replacement and verify that appengine-python-standard no longer emits this warning and remains compatible with Python 3.13.

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