Move Alerting feature to Core

What does this MR do?

Related to #210542 (closed)
Contributes to #42640 (closed)

• Moves Settings > Integrations > Prometheus > Alerts to Core
• Moves projects_with_prometheus_alerts usage data to Core

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Usage data looks like a separate part, but I wasn't sure if it's a good idea to split it and move separately.

Moving usage data before the main feature (Prometheus alerts) to Core might not make sense, because users cannot use the feature yet. On the other hand, moving usage data after release, delays collecting the data of that metric.

So I've put them into the same MR.

Screenshots

There changes is available on "Settings -> Integrations -> Promtheus" page with active Prometheus integration.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• 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.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• ~~Label as security and @ mention @gitlab-com/gl-security/appsec~~
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team