Migration hook fixes (#14174)

* fix offences pre_upgrade hook

* identify source of ensure! failures

* stop migration hooks breaking post migration

* add childbounties storage version

* init child bounties version to zero

* Update frame/child-bounties/src/lib.rs

Co-authored-by: Bastian Köcher <git@kchr.de>

* remove redundant preupgrade version checks

* update test

* fix nom pools v3 migration

* kick ci

* kick ci

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Co-authored-by: Bastian Köcher <git@kchr.de>