Move slackline alerts for SRE on-call from #feed_alerts-general to #production

As part of an on-call onboarding cleanup, I created an MR that started some good discussion -- gitlab-com/runbooks!5168 (comment 1189413493).

Specifically, after some back and forth it looks like the best option is to move the slackline alerts from #feed_alerts-general to #production so that the SREs on-call get better information on the alerts, but don't have to look in a place that is mostly ignored.

@rnienaber confirmed that scalability would own slackline, so this falls under us.