Deprecate ssl_version option

[tsaarni]

This change defaults SSLContext to Python's ssl.create_default_context() and marks ssl_version option as deprecated. The option value will be ignored, and warning will be printed in stderr. The reason for deprecation is that ssl_version option was depending on old method of setting TLS min/max version. Rationale for ignoring the given value is that the method has not worked well anymore with modern Python versions. See discussion thread #2649 (comment).

As an alternative, config file can be used to set the min/max values, for example:

def ssl_context(conf, default_ssl_context_factory):
    import ssl
    context = default_ssl_context_factory()
    context.minimum_version = ssl.TLSVersion.TLSv1_3
    context.maximum_version = ssl.TLSVersion.TLSv1_3
    return context

Additionally, this PR fixes some documentation discrepancies by regenerating settings.rst from config.py. The PR also suggests small modification to docs/gunicorn_ext.py that makes it possible to get sane defaults generated to settings.rst for user, group and chdir options, which otherwise will reset to whatever was effective for the user who regenerated the documentation.

[benoitc]

thanks for the documentation fixes and new default. Merging it as is :)