CMake: Use OpenCV_LIBS instead of OpenCV_LIBRARIES as documented in official OpenCV docs

[traversaro]

OpenCV_LIBS is the variable documented in official OpenCV docs (https://docs.opencv.org/4.x/db/df5/tutorial_linux_gcc_cmake.html). OpenCV_LIBRARIES was a YCM extension, that is going to be deprecated (see robotology/ycm-cmake-modules#434).

[traversaro]

Thanks for opening this pull request! The maintainers of this repository would appreciate it if you would update the release notes by adding a file in doc/release/<target_branch>, based on your changes.

This is an internal change that does not change anything for the external users, so I think it make sense not to document it to avoid increasing the noise in release notes.

[sonarqubecloud]

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