Drop `find_package(libssh CONFIG)` detection method in favour of
the Find module that supports both `pkg-config`, and CMake-native
(since #14555) detection.
This aligns `libssh` detection with other dependencies. It makes the
build honor custom configuration via `LIBSSH_INCLUDE_DIR`,
`LIBSSH_LIBRARY`.
Also enable libssh in a GHA/macos cmake job for build coverage.
Fixing:
- curl-for-win requiring a hack to configure libssh:
4f9acbed92/curl.sh (L255-L263)
- after #14555, GHA/windows gnutls vcpkg job no longer auto-detected
libssh, due to a regression missing to enable libssh when
found via `find_package(libssh CONFIG)`.
Ref: https://github.com/curl/curl/actions/runs/10470138955/job/28994650338
Follow-up to 422696f0a4#14555Closes#14614
576b39b6d0