Breaking API serves no purpose other than to be incompatible with older versions and other implementations that do support priority
(cherry picked from commit bitcoin/bitcoin@870824e919)
Zcash:
* Update the release notes.
* Update the `prioritisetransaction.py` RPC test.
* We don't have BIP 68 or replace-by-fee.
Signed-off-by: Daira Emma Hopwood <daira@jacaranda.org>