Relax OP_RETURN standardness restrictions

Standardness rules in the Bitcoin Core network serve three primary purposes: mitigating Denial of Service (DoS) attacks, providing hooks for future upgrades, and discouraging certain practices. Specifically, there's a rule that limits transactions to no more than one OP_RETURN output, which must also be under 83 bytes. This restriction is designed to discourage the use of blockchain for data storage by making it slightly inconvenient, while still offering an alternative that's less harmful than other methods which are not provably unspendable.

Developers have been finding ways to circumvent these limitations. A noteworthy example is the Citrea bridge named Clementine, which utilizes Taproot outputs—recognized as unspendable—to embed data within its "WatchtowerChallenge" transaction. This innovation highlights how current restrictions on OP_RETURN outputs have spurred creative solutions to bypass standardness rules.

Despite these measures, the effectiveness of such nudges to prevent on-chain data storage has been questioned. The constraints meant to discourage the use of OP_RETURN outputs for storing data have inadvertently encouraged less desirable practices without significantly deterring the intended behavior. In light of this, there's a proposal to eliminate these restrictions, starting with removing the limit on the size of scriptPubKey for OP_RETURN outputs. This initial step aims to prevent encouraging harmful practices. Following this, it's suggested to remove the limitation on the number of OP_RETURN outputs per transaction, further aligning with the proposal's intent to stop promoting detrimental behaviors within the network.