Update Loki serviceMonitor

Improve the Loki serviceMonitor definition

Currently ingested loki metrics have the cluster label added by the chart here

loki_build_info{cluster="loki", container="loki", job="loki/loki-read",service="loki-read"}

This interferes with proper cluster tagging as the default value is loki and not <management_cluster_name>

• This MR drops the cluster label as it's added later by Prometheus via external_labels
• Scrape interval is also increased to 60s to be aligned with other targets