Bump org.openjdk.nashorn:nashorn-core from 15.4 to 15.6 (#15027)

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:development
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

dubbo-cluster/pom.xml

@@ -29,7 +29,7 @@
   <description>The cluster module of dubbo project</description>
   <properties>
     <skip_maven_deploy>false</skip_maven_deploy>
-    <nashorn-core.version>15.4</nashorn-core.version>
+    <nashorn-core.version>15.6</nashorn-core.version>
   </properties>
   <dependencies>
     <dependency>