I backported `find-sibling-file` from Emacs 29+ in 198fe82, but it
passes a third argument (REGEXP) to `file-expand-wildcards` which wasn't
introduced until 29, so users on 28 or earlier would see a
wrong-number-of-arguments error (see #7795).
Rather than backport the entire `file-expand-wildcards`
function (risking other edge cases), I've advised its usage in
`find-sibling-file-search` to behave as it would in 29+ with a non-nil
REGEXP argument.
Amend: 198fe82b6dFix: #7795