Upgrade RubyGems to 2.7.9
This addresses a number of CVEs as described in: https://www.ruby-lang.org/en/news/2019/03/05/multiple-vulnerabilities-in-rubygems/
Note that we currently ship with RubyGems 2.7.6 (/opt/gitlab/embedded/bin/gem -v).
Edited by GitLab Release Tools Bot