Meson: Update deprecated and obsolete function calls
This MR fixes warnings related to:
- an implicit kwarg whose default value depends on the meson version -> it is now explicitly set (see https://github.com/mesonbuild/meson/issues/9300)
- deprecated calls to meson.source_root() updated to meson.global_source_root with no change in behavior
- deprecated call to meson.build_root() updated to meson.current_build_dir(); meson.global_build_root() could be used instead with no change in behavior, but meson.current_build_dir() should not make any difference in this case (as Calendar is a top-level project and not a subproject) and seems more logical to me (see https://mesonbuild.com/Reference-manual_builtin_meson.html#mesonbuild_root)
The above changes require a bump to the minimal meson version from >=0.57.0 to >=0.58.0.
It also removes obsolete join_paths(args) calls and use the / operator instead to be more concise.