Update dependency org.openjdk.nashorn:nashorn-core to v15.6

[renovate]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
org.openjdk.nashorn:nashorn-core	15.5 -> 15.6

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Release Notes

openjdk/nashorn (org.openjdk.nashorn:nashorn-core)

v15.6

Compare Source

#20 JDK-8346302 Fix logging breaking Nashorn initialization

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