[entities] Move CWSource update_config method as a function in 3.15 migration script
This script is the only caller of this method. In the code of the function,
drop "skip_unknown" argument and continue accordingly upon OptionError.
Document: cubicweb-doc
Title: CubicWeb Documentation
Author: Logilab
Abstract: Some base documentation for CubicWeb users and developpers
Section: Apps/Programming
Format: HTML
Index: /usr/share/doc/python-cubicweb-documentation/html/index.html
Files: /usr/share/doc/python-cubicweb-documentation/html/*