Bump org.openjdk.nashorn:nashorn-core from 15.4 to 15.6

Bumps [org.openjdk.nashorn:nashorn-core](https://github.com/openjdk/nashorn) from 15.4 to 15.6.
- [Changelog](https://github.com/openjdk/nashorn/blob/main/CHANGELOG.md)
- [Commits](openjdk/nashorn@release-15.4...release-15.6)

---
updated-dependencies:
- dependency-name: org.openjdk.nashorn:nashorn-core
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

gradle/libs.versions.toml

@@ -87,7 +87,7 @@ jhlabs-filters = { module = "com.jhlabs:filters", version = "2.0.235-1" }
 
 attoparser = { module = "org.attoparser:attoparser", version = "1.3" }
 
-nashorn = { module = "org.openjdk.nashorn:nashorn-core", version = "15.4" }
+nashorn = { module = "org.openjdk.nashorn:nashorn-core", version = "15.6" }
 graalvm-polyglot = { module = "org.graalvm.polyglot:polyglot", version = "23.1.3" }
 graalvm-js = { module = "org.graalvm.polyglot:js", version = "23.1.3" }
 graalvm-jsse = { module = "org.graalvm.js:js-scriptengine", version = "23.1.3" }