BlockId removal: refactor: HeaderBackend::header (#1977)

* BlockId removal: refactor: HeaderBackend::header

It changes the arguments of:
- `HeaderBackend::header`,
- `Client::header`

methods from: `BlockId<Block>` to: `Block::Hash`

This PR is part of BlockId::Number refactoring analysis (paritytech/substrate#11292)

* update lockfile for {"polkadot", "substrate"}

Co-authored-by: parity-processbot <>