Remove deprecated meson.source_root()

This method is deprecated since Meson 0.56.0:
<https://mesonbuild.com/Release-notes-for-0-56-0.html#mesonbuild_root-and-mesonsource_root-are-deprecated>

We could replace it with meson.project_source_root(), but this would
make Meson 0.56 or above mandatory. Since the path in always computed
from the server/ directory, just add '..' to reference the root project
directory.

Refs c456e3826470753bdaefcffc93e9ae1bf25b12f7