This fixes issue #718 (and incidentally fixes #702 too).

The modified `<gsl/util>` imports and various changes from `int` to
`std::size_t` are a consequence of using a newer version of MS GSL. I
changed the dependency version so we can use an upstream CMake package
configuration file which is compatible with the one generated by Conan
2's `CMakeDeps` generator. This should enable us to use the same
`find_package()` commands and target names regardless of whether the
build is running under Conan or not.

This also removes the ability to run Conan from within CMake, because
upstream hasn't released a version of the cmake-conan helper script with
full support for all Conan 2 features yet. (Notably, support for the
`CMakeToolchain` generator is missing.)

… the `package_info` method.

This is because otherwise CMakeDeps incorrectly generates the dependency in the target cmake file as `libcosim::libcosim`. (i.e. <consumer>-Target-release.cmake, see https://docs.conan.io/2/reference/tools/cmake/cmakedeps.html#generated-files). This would not be found via `find_package(libcosim REQUIRED)`.

gcc9 builds are not available from conancenter. Built libraries are uploaded to osp jfrog.