GitLab Runner release manager scheduling

A few releases ago we introduced GitLab Runner (almost) official scheduling for releases, in order to have everything properly documented and allow also people other than @tmaczukin or @ayufan to actually release and deploy the runners on 22nd.

I'd like to see this process picked up also by other developers, the same way it's done for GitLab releases. To make it more official, what about changing https://about.gitlab.com/release-managers/ a little to include also runner release manager? Also, having a trainee is not a bad idea, since probably most of the developers are not familiar with the topic.