Add version file

[ClemensLinnhoff]

For the repository to be a drop-in replacement of the old main repo, a VERSION file is missing.
Add a version file analog to the one in the main repo: https://github.com/OpenSimulationInterface/open-simulation-interface/blob/master/VERSION

[pmai]

There is no need for a VERSION file, as the information from the VERSION file of the main repo is used. The version information is then available via the CMake mechanisms to projects including this one (either via OPEN_SIMULATION_INTERFACE_VERSION or via the verion of the packaged open_simulation_interface-config-version.cmake file).

Directly reading version information from the VERSION file from the outside was never a supported/documented way of interacting with OSI, it was purely for the benefit of OSI's own build infrastructure (CMake and Python setup.py).

Furthermore providing duplicated information runs exactly counter to the idea behind the VERSION file in the first place, which was introduced to ensure that there is only one source of truth for both build systems and osi_version.proto generation.