It only works on BOLT11, and has long been replaced by the more
generic "decode".
Removing it will stop the confusion!
(Note: documentation claims it was introduced in 23.08, but that was
wrong, as it's been in CLN since the beginning).
[ Fixup from: niftynei <niftynei@gmail.com> ]
Fixes: https://github.com/ElementsProject/lightning/issues/6419
Changelog-Deprecated: JSON-RPC: `decodepay`: use `decode`.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>