Use cluster integrations by default [RUN AS-IF-FOSS]

What does this MR do?

Changes references to Clusters::Applications::{ElasticStack,Prometheus} to the corresponding Clusters::Integrations::{ElasticStack,Prometheus} where appropriate.

For context, we are removing one-click applications from GitLab but leaving the integrations for Prometheus and Elastic Stack.

We have code in place that keeps the integration in sync with the corresponding cluster application, and have backfilled the integrations, so we can now use them as a source of truth.

This is not a breaking change (alert updates continue to work for now), but should make it a lot easier to remove the application models when the time comes.

See

• Backfill Prometheus: !61502 (merged)
• Backfill Elastic Stack: !61521 (merged)
• Deprecation Epic: &4280

Screenshots (strongly suggested)

Steps taken:

1. Added a cluster
2. Installed prometheus and elastic stack
   • Pod logs work ✅
   • Metrics work ✅
   • Alerts work ✅
3. Ran cluster.application_elastic_stack.delete and cluster.application_prometheus.delete
   • Pod logs work ✅
   • Metrics work ✅
   • Alerts work ✅

Pod logs	Metrics	Alerts

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.