Disable automatic setup of alerts for Prometheus cluster integration

What does this MR do?

Disables modifying Prometheus to automatically setup alerts for Prometheus cluster integration, as:

• It might not even be installed using Helm.
• That prometheus is not managed by GitLab.
• We don't have access nor permission anyway to modify their Prometheus

This will make the Alerts feature similar to External Prometheus in that it does nothing when the user sets up an Alert (see screenshot)

Builds on top of !55244 (merged) and !59091 (merged)

Related issues: #292460 (closed), #328017 (closed), #327796 (closed)

Screenshots (strongly suggested)

This is where a user clicks to setup Alerts:

This modal will create a PrometheusAlert record in the DB on save.

• For Managed Prometheus, we additionally modify their prometheus to add alerts into their prometheus config
• For External Prometheus, there is no additional step. (i. we don't modify their Prometheus)
• and Prometheus Cluster Integration, there is no additional step (this MR)

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • [-] I have included a changelog entry.
  • I have not included a changelog entry because it is the same feature as !55244 (merged).
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.