fix(chart:k8up): Rename & implement grafanaDashboard.enabled

Summary

Previously, the value was never checked and the ConfigMap was always created. Also rename the parameter to be in line with the other metrics values.

[x] Categorize the PR by setting a good title and adding one of the labels: bug, enhancement, documentation, change, breaking, dependency as they show up in the changelog
[x] PR contains the label area:chart
[x] PR contains the chart label, e.g. chart:k8up
[x] Commits are signed off
[x] Variables are documented in the values.yaml using the format required by Helm-Docs.
[x] Chart Version bumped if immediate release after merging is planned
[x] I have run make chart-docs
[ ] Link this PR to related code release or other issues.