[Vulnerability] jackson-core to version 2.18.6

[dixuswe]

This was flagged in functions-framework-java, I think the dependency is through cloudevents

GoogleCloudPlatform/functions-framework-java#378

https://github.com/advisories/GHSA-72hv-8253-57qq/improve

[akerekes]

sdk-java/formats/json-jackson/pom.xml

Line 34 in 0d96691

<jackson.version>2.15.2</jackson.version>

is where the jackson version is pinned