Spring-ai Use the new vector index creation syntax for Neo4j

To make the Neo4j module more future-proof, this commit replaces the old vector index creation syntax with the new style. Also, the new pattern is in line with the standard Neo4j index creation and supports the IF NOT EXISTS clause to run idempotent. This allows us to remove the preceding call to check if the index exists.

As a consequent, the module will require Neo4j to be at least on version 5.15.

Comment From: tzolov

LGTM, thanks @meistermeier