core/manager: Deprecate StartAuxiliaryScope() method

core/manager: Deprecate StartAuxiliaryScope() method

The method was added with migration of resources in mind (e.g. process's
allocated memory will follow it to the new scope), however, such a
resource migration is not in cgroup semantics. The method may thus have
the intended users and others could be guided to StartTransientUnit().

Since this API was advertised in a regular release, start the removal
with a deprecation message to callers.
Eventually, the goal is to remove the method to clean up DBus API and
simplify code (removal of cgroup_context_copy()).

Part of DBus docs is retained to satisfy build checks.