Introduce more strict runner release process

We currently have GitLab Runner that is release quite different to release process of GitLab: https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/blob/master/docs/release_process/README.md.

The release process should be equal to release process of GitLab with all it dates and should also be handled by Release Managers that release a GitLab releases.

For reference: https://docs.gitlab.com/runner/release_process/README.html, https://about.gitlab.com/release-managers/, https://gitlab.com/gitlab-org/release-tools/tree/master.

I propose to start doing that with 9.1 release of Runner, basically after the releasing 9.0 to shorten the merge window for 9.1 features to 7th.