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]>

cj-bitcoinj-dsl-js/build.gradle

@@ -10,7 +10,7 @@ tasks.withType(JavaCompile).configureEach {
 
 dependencies {
     api project(':cj-btc-jsonrpc')
-    implementation 'org.openjdk.nashorn:nashorn-core:15.4'
+    implementation 'org.openjdk.nashorn:nashorn-core:15.6'
 
     testImplementation "org.slf4j:slf4j-jdk14:${slf4jVersion}"       // Runtime implementation of slf4j
 }