Changes cron schedule for `geo_prune_event_log_worker` from
`0 */6 * * *` to `0 */2 * * *`.

It reduces the average of 600k events deleted every 6h, to 200k events
deleted every 2h:
https://gitlab.com/gitlab-com/infrastructure/issues/4231#note_79874454

Closes gitlab-com/infrastructure#4231.