[server] deprecate the old multi-source system
It is removed in 3.19.
Closes #2919299
What's new in CubicWeb 3.18?
============================
New functionalities
--------------------
* add a security debugging tool
(see `#2920304 <http://www.cubicweb.org/2920304>`_)
API changes
-----------
* not really an API change, but the entity permission checks are now
systematically deferred to an operation, instead of a) trying in a
hook and b) if it failed, retrying later in an operation
* The default value storage for attributes is no longer String, but
Bytes. This opens the road to storing arbitrary python objects, e.g.
numpy arrays, and fixes a bug where default values whose truth value
was False were not properly migrated.
Deprecation
---------------------
* the old multi-source system
Deprecated Code Drops
----------------------
* ``ldapuser`` have been dropped; use ``ldapfeed`` now
(see `#2936496 <http://www.cubicweb.org/2936496>`_)
* action ``GotRhythm`` was removed, make sure you do not
import it in your cubes (even to unregister it)
(see `#3093362 <http://www.cubicweb.org/3093362>`_)
* all 3.8 backward compat is gone
* all 3.9 backward compat (including the javascript side) is gone