The tx verifier does not enforce using the correct consensus branch ID in the SIGHASH computation

[upbqdn]

When computing the SIGHASH of V5 txs, the tx verifier picks the consensus branch ID from the nConsensusBranchId field of the tx. If the field is set to a consensus branch ID corresponding to NU5 and NU6 is active, the SIGHASH computation doesn't follow this consensus rule:

[NU6 only] All transactions MUST use the NU6 consensus branch ID 0xC8E71055 as defined in ZIP-253.

This bug was effectively mitigated in #9063 by checking the nConsensusBranchId field prior to the SIGHASH computation, so the tx verifier won't accept NU5 transactions after NU6 activation anymore. However, we should fix the computation itself so that it adheres to the consensus rule stated above.