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]>

build.gradle

@@ -29,7 +29,7 @@ dependencies {
 
     // was removed in jdk 15
     if(JavaVersion.current() >= JavaVersion.VERSION_15) {
-        implementation 'org.openjdk.nashorn:nashorn-core:15.4'
+        implementation 'org.openjdk.nashorn:nashorn-core:15.6'
     }
 }