Set PM for the Retention group to Mike Karampalas
This was suggested by @sfwgitlab in !253 (comment 203137079).
We need Mike's GitLab username to do that, so we're waiting for it. It should be available in his onboarding issues as soon as it will be created: https://gitlab.com/gitlab-com/people-ops/employment/issues?scope=all&utf8=✓&state=opened&search=Karampalas