cubicweb #3753250 [schema] stop using RQLUniqueConstraint [validation pending]

the remaining use (in workflow entities) can be replaced by __unique_together__

prioritynormal
typeenhancement
done in3.20.0
load0.200
load left0.000
closed by#f0363da0b5a0 [schema] stop using RQLUniqueConstraint (closes #3753250)
patch[schema] deprecate RQLUniqueConstraint (closes #3753250) [applied]