cubicweb #3683640 issues with defaultval in 3.18 migration [validation pending]

the runtime schema object is not properly updated, so later changes in the same migration session get unexpected results.

priorityimportant
typebug
done in3.18.4
load0.500
load left0.000
closed by#7e4897901c64 [migration] Improve update of in-memory schema during 3.18 CWAttribute.defaultval change
patch[migration] Improve update of in-memory schema during 3.18 CWAttribute.defaultval change [applied]