Update prometheus-client-mmap to v0.12.0

This removes the deprecated rb_safe_level() and rb_secure() calls. Ruby
2.7 removed the the taint checking mechanism because "the taint
mechanism is too expensive to maintain for the merit of it.":
https://bugs.ruby-lang.org/issues/16131

This pulls in
https://gitlab.com/gitlab-org/prometheus-client-mmap/-/merge_requests/53