monitoring: zoekt: fix heatmap configuration

Created by: ggilmore

@stefanhengl and I realized that the heat maps on the Zoekt dashboards were broken. They didn't include data-points (like lengthy indexing durations) that we knew existed (we could see them in the logs).

It turns out that we weren't using the right Grafana incantations to inform it that the data source that we're using for the heatmap is already bucketed - it doesn't need to recreate its own buckets. We fixed that configuration, and also changed the heatmap to use a higher-contrast color scheme.

Before:

After:

See this blog post for the guide that we followed.

Test plan

We used the local monitoring stack with sourcegraph.com's Prometheus instance to see that these changes fixed the issue.