[kube-prometheus-stack] Add grafana persistence values

[QuentinBisson]

What this PR does / why we need it:

Which issue this PR fixes

This PR adds the persistence values for the grafana dependency as using persistence is a regular use case and it makes it easier to onboard with this helm chat

Special notes for your reviewer:

Checklist

• [x] DCO signed
• [x] Chart Version bumped
• [x] Title of the PR starts with chart name (e.g. [prometheus-couchdb-exporter])

[QuentinBisson]

@monotek how about a comment explaning that there are a lot of other things to configure described in the grafana helm chart like persistence and so forth ?

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Any further update will cause the issue/pull request to no longer be considered stale. Thank you for your contributions.

[stale[bot]]

This issue is being automatically closed due to inactivity.