Remove prometheus integrations from devops score

What does this MR do and why?

context:

!199066 (merged) removes the projects_prometheus_active metric, which was historically a contributor to an instance's devops score. The corresponding prometheus feature was deprecated in %15.0 & removed from the product in %16.0, so we should have removed it from the score years ago. There is no intended replacement at this time.

this mr:

• Removes the Monitoring section from the devops score (as prometheus integrations were the only contributing feature)
• Removes not-null constraints from prometheus-related columns in the conversational_development_index_metrics tables

Note: This should merge after Remove Integrations::Prometheus-based service p... (!199066 - merged)

References

• Broader issue: https://gitlab.com/gitlab-org/gitlab/-/issues/409734+
• Metric removal issue: Remove `counts.projects_prometheus_active` metr... (#557479 - closed)
• Metric removal MR: Remove Integrations::Prometheus-based service p... (!199066 - merged)
• Version App removal MR: Remove deprecated projects_prometheus_active me... (gitlab-org/gitlab-services/version.gitlab.com!234 - merged)

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