generate-docs-nm-settings-docs-gir: Port to libgirepository-2.0

pygobject 3.52 has switched 1 to using libgirepository-2.0 which comes from glib itself now, rather than the 1.0 which came from gobject-introspection.

This means that it fails to load the incompatible "GIRepository 2.0" and thus must be ported to 3.0 (which is provided by libgirepository-2.0).

Migration guide is here 2

This commit adds suppport for importing with "gi.require_version("GIRepository", "3.0") and falling back to the existing "GIRepository 2.0" if not found.