Companion for substrate#8724 (#2994)

* runtime: create migrations for grandpa storage prefix

* runtime: address review comments

* runtime: update spec_version

* westend: fix incorrect merge

* runtime: change to v3.1 from v4 for grandpa migrations

* cargo.lock: manually specify pallet-grandpa in lock file

* cargo.lock: fix typo

* update substrate

* update substrate

Co-authored-by: André Silva <[email protected]>