Change group to `group::monitor` in all feature flags owned by Monitor

What does this MR do?

After group::apm was removed group::health was renamed to group::monitor.

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