Fix pgvector docker image pull by using postgres 15 tag

We were previously relying on an older, unmaintained version of
pgvector docker image, ankane/pgvector.

Moving to new docker image requires selecting from tags based on the
pg major version (14, 15, 16 or 17).

This change uses pg15 tag to resolve image pull.
Note: we use postgres 15 for khoj docker images currently

Fixes #1154

docker-compose.yml

@@ -1,6 +1,6 @@
 services:
   database:
-    image: docker.io/pgvector/pgvector
+    image: docker.io/pgvector/pgvector:pg15
     environment:
       POSTGRES_USER: postgres
       POSTGRES_PASSWORD: postgres