Drop deprecated metrics category tables

What does this MR do and why?

Changes: Drops all of the now-obsolete tables related to the deprecated Category:Metrics features. These tables include:

• issues_prometheus_alert_events
• issues_self_managed_prometheus_alert_events
• prometheus_alert_events
• self_managed_prometheus_alert_events
• prometheus_alerts
• prometheus_metrics

This MR is broken into one commit per table for reviewability. Review commit-by-commit here! The first commit is empty but has an appropriate commit message for the whole MR to use when merging.

Why a single MR? A couple reasons:

1. Dropping these tables is order-dependent; a single MR ensures the order without heaps of rebases between MRs
2. The migrations for each table are nearly identical, so any feedback can be applied more uniformly across each migration
3. Distributing these reviews out over multiple people/days increases the costs of context-gathering/context-switching by much more than the increased effort of reviewing

Relationship between tables:

graph LR;
  subgraph KEEP THESE!
    issues
    projects
    environments
    alert_management_alerts
  end
  subgraph DROP THESE!
    direction LR
    issues_prometheus_alert_events
    issues_self_managed_prometheus_alert_events
    prometheus_alert_events
    self_managed_prometheus_alert_events
    prometheus_alerts
    prometheus_metrics
    subgraph self-managed tables
      issues_self_managed_prometheus_alert_events-->self_managed_prometheus_alert_events;
      self_managed_prometheus_alert_events-->projects;
      self_managed_prometheus_alert_events-->environments;
      issues_self_managed_prometheus_alert_events-->issues
    end
    subgraph gitlab-managed tables
      issues_prometheus_alert_events --> prometheus_alert_events;
      prometheus_alert_events-->prometheus_alerts;
      prometheus_alert_events-->projects;
      prometheus_alerts-->prometheus_metrics;
      issues_prometheus_alert_events-->issues;
      alert_management_alerts --> prometheus_alerts;
      prometheus_alerts-->projects;
      prometheus_alerts-->environments;
      prometheus_metrics-->projects;
    end
  end

Context: This MR cleans up a portion of dead code related to the removal of devopsmonitor Category:Metrics features, previously owned by grouprespond. The Metrics Dashboard & Prometheus Metrics/Alerts (used for monitoring apps deploying using gitlab) were deprecated in 14.7 and removed from the product in 16.0.

• grouprespond related issue: Metrics: Delete models & database tables/columns (#397142)
• Monitor:Metrics removal epic: Monitor: Metrics Deprecation and Removal (&10107)
• Reference MR with all related removals: Draft: Remove all remaining metrics dashboard-r... (!160325 - closed)
• Metrics removal is blocking cells: Drop old unused tables for prometheus alerts (#461838 - closed)

MR acceptance checklist

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