refactor(metrics-catalog): mimir datasource helper
What
Update Mimir default datasource to use Mimir helper for Mimir -
prefix.
Why
Helper was introduced in !7310 (merged).
Snapshot
https://dashboards.gitlab.net/dashboard/snapshot/2B4wg9wlL3l13qGVRJWwgkP48ffsPm5p?orgId=1
Edited by Chance Feick