[Sequencer/RPC] consolidate gasless flags

[hexoscott]

There are two flags in cdk-erigon to control gasless networks:

• zkevm.gasless
• zkevm.allow-free-transactions

These two flags will never be set differently from each other and should be consolidated to remove confusion.

[hexoscott]

there will be lots of nodes running cdk-erigon already so if we can make this a non-breaking change somehow that would be ideal.