Conflicting HealthHandler methods and missing import

[Dheavyman]

The _check_vector_store_health method in HealthHandler appears two times which means the second is overwriting the first one. This causes an unexpected outcome during health check.
You can find the conflicts in HealthHandler here(line 84) and here(line (115)

• Fix method name conflict

Also there is a missing import statement for datetime used here and I suggest we avoid the deprecated utcnow()

• Add needed imports

from datetime import datetime, timezone

• Replace deprecated utcnow() function

-   "timestamp": datetime.utcnow().isoformat(),
+   "timestamp": datetime.now(timezone.utc).isoformat(),