Follow up work on `TransactionExtension` - fix weights and clean up `UncheckedExtrinsic` (#6418)

Follow up to https://github.com/paritytech/polkadot-sdk/pull/3685
Partially fixes https://github.com/paritytech/polkadot-sdk/issues/6403

The main PR introduced bare support for the new extension version byte
as well as extension weights and benchmarking.

This PR:
- Removes the redundant extension version byte from the signed v4
extrinsic, previously unused and defaulted to 0.
- Adds the extension version byte to the inherited implication passed to
`General` transactions.
- Whitelists the `pallet_authorship::Author`, `frame_system::Digest` and
`pallet_transaction_payment::NextFeeMultiplier` storage items as they
are read multiple times by extensions for each transaction, but are hot
in memory and currently overestimate the weight.
- Whitelists the benchmark caller for `CheckEra` and `CheckGenesis` as
the reads are performed for every transaction and overestimate the
weight.
- Updates the umbrella frame weight...