`find_package(CURL CONFIG)` provides `CURL_VERSION` from the upstream
cmake package version file.
Upstream curl commit `699ac9430c` (cmake: publish/check supported
protocols/features via `CURLConfig.cmake`, 2024-12-29) extends the
upstream cmake package to provide our old `CURL_VERSION_STRING`.
Provide both names from CMake's own module to aid transition.
Fixes: #26634