cubicweb #4619278 migration: be robust against duplicate unique together constraints [validation pending]

Duplicates here will crash the migration.

prioritynormal
typebug
done in3.18.7
load0.100
load left0.000
closed by#8f7291354095 [schemaserial] be robust against duplicated CWUniqueTogetherConstrain entities arising from bug in the pre 3.18 era
patch[schemaserial] be robust against duplicated CWUniqueTogetherConstrain entities arising from bug in the pre 3.18 era [applied]