cubicweb #2567712 deprecate ldapuser source [validation pending]

The last production sites are going ldapfeed and experience shows that bitrot already crept in (once) in the 3.15 timeframe.

prioritynormal
typeenhancement
done in3.17.0
load0.100
load left0.000
closed by#bda63114015c [ldapuser] Deprecated the source
patch[ldapuser] Deprecated the source [applied]