NO-JIRA: Use maven for IDE scripting

[Skylli202]

Motivation

The UsingAnIdeToWriteScripts example was based on an outdated Gradle configuration and Flex SDK version. This made the project harder to import into IntelliJ IDEA and no longer reflected the Java and Groovy environment used by JAE (flex-jobasyncexecutor-service) to execute Flex scripts.

Changes

• Migrates UsingAnIdeToWriteScripts from Gradle to Maven.
• Updates the example to Java 21, matching the JAE build and runtime.
• Updates flex-sdk-external to <new SDK version>.
• Uses the Groovy version published transitively by the Flex SDK, which is aligned with JAE’s parent-managed runtime.
• Replaces the Spock test with JUnit 5 and Mockito while keeping the test written in Groovy.
• Registers Groovy source directories for Maven and IntelliJ IDEA.
• Updates the documentation with IntelliJ setup, Maven commands and Nexus mirror configuration.
• Leaves the separate ExternalScripts Gradle example unchanged.

The result is a simpler IDE-ready example that provides Flex SDK code completion and tests scripts against dependencies compatible with the environment in which JAE executes them.