[V124-specific] Set StatusSince upon change from RetiringVault to InactiveVault
[V124-specific]
Intended to close #1704 (closed)
'RetiringVault -> InactiveVault height isn't recorded in StatusSince'
, specifically so that if there is an InactiveVault inbound,
it can be directly at what height that vault ceased being a RetiringVault and became an InactiveVault.
(One block earlier than that can still be used to see when it ceased being an ActiveVault and became a RetiringVault.)