Fix installation of dependencies on Debian/Ubuntu via apt

[traversaro]

This PR fixes the installation of meshcat-cpp on Debian/Ubuntu via apt:

• Switch to get uSockets/uWebSockets via FetchContent and build them via CMake, ensuring better portability (for this reason, drop git submodules)
• Switch to call FetchContent via URL instead of GIT, for some reason (not sure why, it can be WSL2-related) using FetchContent via GIT on msgpack-cxx, usockets and uwebsockets was taking a really long time (like 1 minute for repo).
• Bump msgpack-cxx to version 6.0.0 that has some breaking changes related to the C++ package name and the CMake target name (see https://github.com/msgpack/msgpack-c/issues/1043)
• I also deleted the file cmake/MeshcatCppDeps.cmake as it was unused