Reduce rate of RepositoryUpdateMirrorWorker jobs

Introduction

Currently by default on GitLab.com, every mirrored repository is checked for updates twice an hour.

This generates about 200k updates jobs per hour.

These jobs generate enormous amounts of pressure on GitLab.com's Gitaly servers.

Proposal

Reduce the rate of RepositoryUpdateMirrorWorker by 50%, so that on average, each repository is updated once an hour.

It's not clear what we would need to change in these settings (pictured below) to reduce this by 50%