Set PromtailDown alert to not page out of business hours

[hervenicol]

This PR prevents PromtailDown from paging out of business hours.
This will allow us to take some time to tune and stabilize the alert, then we can change this back when the alert does not page that often.

Checklist

• Update CHANGELOG.md
• Add Unit tests
• Follow Alert structure
• Consider creating a dashboard (guidelines) (if it does not exist already) to help oncallers monitor the status of the issue.
• Request review from oncall area, as well as team (e.g: oncall-kaas-cloud GitHub group).