[fix] Bound staking ledger correctly with MaxUnlockingChunks from configuration (#12343)

* used maxunlockingchunks from config

* mhl MaxUnlockingChunks

* no migration needed

* changes as per requested

* fmt

* fix tests

* fix benchmark

* warning in the doc for abrupt changes in the config

* less unnecessary details in the test

* fix tests

Co-authored-by: mrisholukamba <[email protected]>
Co-authored-by: parity-processbot <>