Inaccessible data stored at session pallet

[kianenigma]

paritytech/substrate#9878 was added to both chains seemingly around 9.12 release, yet we somehow missed its companion. Luckily, this is now all good since after 28 days, the pallet is fully functioning in its new storage prefix (Historical).

The problem is that the two storage items of this pallet still live under the old prefix as well, with their data being stale and not really useful anymore.

We need a migration that goes and cleans everything under Session::HistoricalSessions and Session::StoredRanges.