DEVX-2465: change thresholds to mirror basic cluster limits, update panels, and image

[awalther28]

This PR proposes incorporating Confluent "Basic" cluster limits as the thresholds for the key metrics highlighted at the top of the dashboard. Basic cluster and Standard cluster limits are very similar, the key differences being Standard allows infinite storage in AWS (5TB on basic) and max 500 partition creation/deletion over 5 minute (250 on basic).

Key metrics proposed to be highlighted on the top of the dashboard with thresholds at 80% and 90% of the max value :

• Partitions--Max 2048
• Total client connections--Max 1000
• Ingress--Max 100 MBps
• Egress--Max 100 MBps
• Requests--Max 1500 per second
• Storage--Max 5 TB
• Partition creation and deletion--Max 250 per 5 minute period
• api latency (already existed)

On each new/upgraded panel, a description was added. Rates were introduced where needed to properly compare metrics to Confluent's cluster limits.

[Dabz]

Thanks a lot @awalther28 for doing this ;) I imported the dashboard on my local Grafana, everything's work perfectly :+1:

I will squash & merge on master. Thanks for the PR!