alerts: add CeleryNotReporting paging alert

[xvello]

Description

Add CeleryNotReporting to page us when Celery metrics are not properly reported.

Merging this is blocked on us moving from prometheus-pushgateway to direct VictoriaMetrics push. After that change, we should be able to use the absent_over_time function properly.

⚠️ Let's not forget to bump the chart version before merging.

Type of change

• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

How has this been tested?

Checklist

• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works