Use HttpIntegrations to monitor demo projects for monitor team

What does this MR do?

Related issue: #255501 (closed)

The monitor team has a couple of demo projects used to test monitor features. When these projects have problems with prometheus, an alert is triggered using the generic alert service associated with the project. With the transition from AlertsService to HttpIntegrations, we need to transition over this monitoring tooling as well.

This MR swaps these health checks from alerting with the legacy alert service integration over to using the new HTTP integration. As all legacy integrations were migrated to the new format already, there should be no disruption to the behavior.

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