Stop liveness check returning incorrect data

What does this MR do?

This changes the response body of the liveness check so that less misleading to users. Related issue: #31033 (closed)

The existing response wrongly indicated that each of the services were checked, when it fact they were hardcoded to success. The liveness check is intended to indicate if the service is alive. It does this by returning a 200 response, after connecting to the DB (as part of the Rails request lifecycle).

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content