Remove outdated installation methods [skip ci]
Remove outdated installation methods [skip ci]

In particular, don't recommend Conan with CMake because it is messed up. (Conan chooses to ignore our "gsl-lite-config.cmake" and assembles its own find module, which of course knows nothing about extra targets like `gsl::gsl-lite-v1`.)

martinmoene/gsl-lite,martinmoene/gsl-lite,martinmoene/gsl-lite