Deprecate `__CPROVER_allocated_memory`

The plan for `__CPROVER_allocated_memory` seems to be to remove it in PR #6748 and replace it with a command line option in #6747 for the next major release of cbmc. Therefore we should emit a suitable warning about it being deprecated when it is found in user code. That way we can discourage new uses of it and any existing users will be suitably warned that it will need to be replaced.
