cubicweb #4707737 [schema] deprecate RQLUniqueConstraint [open]

Which cannot work in a distributed deployment.

(also most of the use cases can be covered by a multicolumns unique constraints).

prioritynormal
typeenhancement
done in<not specified>
load0.200
load left0.200
closed by<not specified>
patch[schema] deprecate RQLUniqueConstraint (closes #3753250) [applied]