bip54: exempt genesis from coinbase checks

[nervana21]

The BIP54 reference implementation skips genesis when checking the new coinbase locktime and sequence rules. However this wasn't specified in the BIP.

Update the BIP to reflect that genesis is exempt from these checks.

[fametrano]

Concept ACK. All 7 cases in bip-0054/test_vectors/coinbases.json start from the mainnet genesis block. Its coinbase has nLockTime 0 and nSequence 0xffffffff, so the rule as written makes all of them invalid. bitcoin/bitcoin#35793 also skips the check at height 0.

The timewarp rule has the same gap: at N=0 there is no T_N−1, and timestamps.json also starts from genesis. One sentence could exempt genesis from both rules.