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.
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