QBFT doesn't support time-based block transitions for `blockperiodseconds`

[matthew1001]

Steps to Reproduce

1. Create a QBFT chain with a time-based fork enabled (e.g. shanghaiTime: 0
2. Add a QBFT transition to a future timestamp, changing the blockperiodseconds
3. After the fork timestamp, blocks continue to be produced at the old blockperiodseconds but are validated against the new value, so validation fails.