[stable/grafana] Setting persistence size

[southwolf]

What this PR does / why we need it:

The persistence.size was commented out, so users must set it manually if they want use persistence. This PR uncommented it to save the extra steps

Which issue this PR fixes

• fixes [stable/grafana] Documentation on default persistence values is incorrect #5102

Special notes for your reviewer:

Checklist

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

• DCO signed
• Chart Version bumped

[k8s-ci-robot]

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[sameersbn]

Thanks for the contribution. This issue was resolved in #10307