[grafana] cleanup dashboards

[puckpuck]

Changes

Replaces #2078

• uses Otel Collector dashboard from https://grafana.com/grafana/dashboards/15983-opentelemetry-collector/
• updates to Demo dashboard including a service map
• updates all dashboards to reflect a shorter time window (15-m)
• removes Otel Collector flow dashboard
• configured uid for opensearch datasource in Grafana
• set a home folder in Grafana
• disables all Prometheus scraping
• updates Otel Collector self telemetry to use OTLP
• configures memory_limiter for all Otel Collector pipelines

Merge Requirements

For new features contributions, please make sure you have completed the following
essential items:

• CHANGELOG.md updated to document new feature additions
• Appropriate documentation updates in the docs
• Appropriate Helm chart updates in the helm-charts

Maintainers will not merge until the above have been completed. If you're unsure
which docs need to be changed ping the
@open-telemetry/demo-approvers.

[julianocosta89]

Left a minor comment, but not a blocker, LGTM