Make it always a number; this makes the JSON request specification
simpler. We allowed a number since v0.10.1.
(reserve=True is the default anyway, so usually it can be omitted:
reserve=False becomes reserve=0).
Changelog-Deprecated: JSON-RPC: `fundpsbt`/`utxopsbt` `reserve` must be a number, not bool (for `true` use 72/don't specify, for `false` use 0). Numbers have been allowed since v0.10.1.