network_parameters: change bip21 schema liquidtestnet->liquidnetwork

[RCasatta]

The bip21 schema doesn't need to convey the network, indeed for bitcoin we have only bitcoin: as schema for every network (mainnet, testnet, signet, regtest)

This is indeed a breaking change and the liquidtesnet schema is used in the wild, however, since it's breaking only for testnet networks, I think it makes sense to change it.

@angelix @lvaccaro @LeoComandini @jgriffiths @jkauffman1 @domegabri @whitslack @alessandro-saglimbeni

related ElementsProject/elements#805

[LeoComandini]

ACK 71937f6

I would add a changelog entry though.

[whitslack]

Concept NAK. Bitcoin's overloading of the bitcoin: scheme is arguably a design oversight with real drawbacks for users. Consider that quite often wallet software will come packaged as separate apps for mainnet and testnet implementations. Then, users are forced either to avoid setting a default handler for BIP21 URIs (and thus always needing to choose a handling app every time they invoke a URI) or to choose a default app and lose the ability to invoke URIs intended for other networks. Using distinct URI schemes allows each network to have an appropriate app configured to handle its own relevant URIs.

[LeoComandini]

ACK 381f03c, we should do the same as bitcoin