devel/rubygem-prometheus-client-mmap: update to 1.2.8

Required for gitlab 17.9

Changes: https://gitlab.com/gitlab-org/ruby/gems/prometheus-client-mmap/-/blob/1-1-stable/CHANGELOG.md
(cherry picked from commit 7c92d39c7b922752c6baf671cbdb90af8c35eb27)