Fix `gdk doctor` when pgvector enabled without the embeddings database

What does this merge request do and why?

Previously for the gdk doctor PostgreSQL diagnostic we checked if the embeddings database were enabled, but we really should check whether pgvector is enabled. The instructions in https://docs.gitlab.com/ee/development/ai_features.html#set-up-the-embedding-database have users enable pgvector first, so if you did a gdk reconfigure before enabling the embedding database gdk doctor would not perform the check introduced in !3149 (merged).

How to set up and validate locally

Not sure if there's a good way to cause a mismatch in !3149 (merged).

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.