Substitute `enable_rbf` in `TxBuilder` with `disable_rbf`

[danielabrozzoni]

i.e., maybe all our txs should be RBF by default :)

[notmandatory]

Concept ACK, seems to be direction bitcoin core is going with bitcoin/bitcoin#25353. Even with all the controversy I think best practice from a general wallet point of view is to enable RBF so fees can easily be bumped when needed.

[notmandatory]

Since this is a breaking change we're going to hold off on this until 1.0 since that is a more obviously breaking release.

[notmandatory]

A small change but I think we should push to 2.0 milestone.