[stable/prometheus] Call for maintainers

[cstaud]

Is this a new chart

No

What this PR does / why we need it:

Add myself as OWNER, @gianrubio seems do be not active anymore.

Which issue this PR fixes

(optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged)

• fixes Call for maintainers for charts with a single OWNER #13867
• fixes Call for maintainers for charts with no OWNERS #13869

Special notes for your reviewer:

Checklist

[Place an '[x]' (no spaces) in all applicable fields. Please remove unrelated fields.]

• DCO signed
• Chart Version bumped
• Variables are documented in the README.md
• Title of the PR starts with chart name (e.g. [stable/mychartname])

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: cstaud
To complete the pull request process, please assign gianrubio
You can assign the PR to them by writing /assign @gianrubio in a comment when ready.

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Needs approval from an approver in each of these files:

• stable/prometheus/OWNERS

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