Dashboards: do not use red color for non failures in overview dashboard

[pracucci]

What this PR does

I received a feedback to not use red color for non failure series in the "Mimir / Overview" dashboard. I think this is a good feedback we could apply to all dashboards but we to have to start somewhere (overview dashboard). This PR proposes a way to do it.

This PR is draft because it builds on #3245.

Preview of updated panels:

Which issue(s) this PR fixes or relates to

N/A

Checklist

• [ ] Tests updated
• [ ] Documentation added
• [x] CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]