fix(wallet): off-by-one error checking coinbase maturity in optional UTxOs

fix(wallet): off-by-one error checking coinbase maturity in optional UTxOs

The `preselect_utxos` method has an off-by-one error that is making the
selection of optional UTxOs too restrictive, by requiring the coinbase
outputs to surpass or equal coinbase maturity time at the current height
of the selection, and not in the block in which the transaction may be
included in the blockchain.

The changes in this commit fix it by considering the maturity of the
coinbase output at the spending height and not the transaction creation
height, this means, a +1 at the considered height at the moment of
building the transaction.