[obs] remove noisy GitpodWorkspaceStuckOnStoppedMk2 (#18998)

Pods stuck in Stopped are removed after the 30m grace period: https://github.com/gitpod-io/gitpod/blob/c346773e50f4a15d9af4a663c5f1004c962f4d58/components/ws-manager-mk2/controllers/create.go#L54

Author: kylos101

operations/observability/mixins/workspace/rules/central/workspaces.yaml

@@ -14,18 +14,6 @@ spec:
 
   - name: workspace-alerts
     rules:
-    - alert: GitpodWorkspaceStuckOnStoppedMk2
-      labels:
-        severity: warning
-        team: engine
-      for: 20m
-      annotations:
-        runbook_url: https://github.com/gitpod-io/runbooks/blob/main/runbooks/GitpodWorkspaceStuckOnStopped.md
-        summary: Workspaces are stuck in STOPPED phase on cluster {{ $labels.cluster }}.
-        description: '{{ printf "%.2f" $value }} regular workspaces are stuck on stopped for more than 20 minutes. Current status: {{ $labels.reason }}'
-      expr: |
-        sum(gitpod_ws_manager_mk2_workspace_phase_total{phase="Stopped", cluster!~"ephemeral.*"}) > 5
-
     - alert: GitpodWorkspaceStuckOnStoppingMk2
       labels:
         severity: critical