Update grafana chart version to 6.9.1

What does this MR do?

This MR updates the grafana chart version to 6.9.0 to resolve the rbac.authorization.k8s.io/v1beta1 deprecation issues and also benefit from the latest grafana features and to match up with the Omnibus version which was upgraded to 7.5.5

Limitation / Issues

• As per 6.0.0, version > 6.0.0 requires helm Helm >= 3.1.0 and unfortunately this issue is not fixed in version < 6.0.0 so we may have to wait for 14.0 milestone.
• Another issue is, as from version 5.0.0 the selector labels of the chart have changed, the recommendation is to use --force which may not be appropriate for our case. A solution will be to delete the previous deployment so would be an additional step to upgrade. (I tried to check but cannot find a way to apply --force for the specific dependencies as this would also work)

kubectl delete deployments.apps gitlab-grafana

If this is not done, the upgrade fails with the following error

Error: UPGRADE FAILED: cannot patch "gitlab-grafana" with kind Deployment: Deployment.apps "gitlab-grafana" is invalid: spec.selector: Invalid value: v1.LabelSelector{MatchLabels:map[string]string{"app.kubernetes.io/instance":"gitlab", "app.kubernetes.io/name":"grafana"}, MatchExpressions:[]v1.LabelSelectorRequirement(nil)}: field is immutable

Related issues

Closes #2714 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for omnibus-gitlab opened