Prometheus: update chart origin location

What does this MR do?

Move to using the upstream origin of prometheus-community versus helm/stable ahead of deprecation.

Now sourcing per https://artifacthub.io/packages/helm/prometheus-community/prometheus/10.0.0

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added