This commit fixes the heuristic we use for identifying the party
that broadcast a Simple Taproot Channel commitment transaction.
Prior to this change we checked if the last script element was an
OP_DROP. However, both the local and remote commitment outputs
have an OP_DROP at the end.
The new approach checks the resolver's SignDescriptor and compares
that key to the keys in the channel's local ChannelConfig. If the
key is the delay key, we know that it is our commitment transaction.
09f5e08d32