Upstream CURL provides imported target `CURL::libcurl`. Rename the
target added by `FindCURL` to match. We don't need compatibility with
the old name because it has never been in a CMake release (except a 3.12
release candidate).
Suggested-by: Jakub Zakrzewski <slither.jz@gmail.com>
Acked-by: Rolf Eike Beer <eike@sf-mail.de>
Fixes: #18091