[Alert KPIs] Aert KPI tech debt

[christineweng]

The kpi visualizations were introduced in 8.7. As we are now in 8.16, we should remove the feature flags and clean up the code.

Cypress coverage
Currently the alert table tests relies on Counts tab to check alert count. We should expand the coverage to other tabs as well:

• Basic cypress for Overview, Trend, Counts, Tree map
• Status change from alert table for row actions, bulk actions and take actions from grouping
• Status change from status change in alert flyout

Tasks

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1. [Security Solution] Reorganize kpi visualization folders #189234
   backport:skip release_note:skip v8.16.0 +3
   
2. [Security Solution] Remove alerts page KPI feature flag #189424
   Team:Threat Hunting Team:Threat Hunting:Investigations backport:skip release_note:skip v8.16.0 +5
   
3. [Security Solution][Alert Details] Remove alert type charts feature flag #189437
   Team:Threat Hunting Team:Threat Hunting:Investigations backport:skip release_note:skip v8.16.0 +5
   
4. [Security Solution][Alert Details] Alerts KPI clean up #189837
   Team:Threat Hunting Team:Threat Hunting:Investigations backport:skip release_note:skip v8.16.0 +5
   
5. [Security Solutions][Alert Details] Add cypress coverage for kpi charts #189758
   Team:Threat Hunting Team:Threat Hunting:Investigations backport:skip release_note:skip v8.16.0 +5
   

[elasticmachine]

Pinging @elastic/security-solution (Team: SecuritySolution)