Upgrade Graddle Wrapper, JUnit, Approvaltests, and Dom4j

[ibanFR]

This PR upgrades build and test dependencies and modernizes the test configuration to JUnit 5.

This PR upgrades three dependencies used by the project: junit, approvaltests, and dom4j. The updates are intended to bring the project up-to-date with newer, more secure, and better-supported versions of these libraries. In addition to the dependency bumps, the Gradle Wrapper has also been updated to a newer version to ensure reproducible builds and compatibility with the updated dependencies.

What changed

Upgrade Gradle Wrapper

• Update the Gradle wrapper to use Gradle 9.2.0.

Migrate to JUnit 5 (Jupiter)

• Adopt the JUnit Platform with useJUnitPlatform() in Gradle.
• Use the official JUnit BOM (org.junit:junit-bom) to manage test dependency versions.
• Replace legacy org.junit imports with org.junit.jupiter equivalents.

Upgrade other dependencies

• org.dom4j:dom4j → 2.1.4 (latest in the 2.1.x line).
• com.approvaltests:approvaltests → 25.7.0 (latest stable release).

Why

• Build Tooling: Upgrading Gradle lets us use newer plugin APIs, improved performance, and long-term tooling support.
• Security: Older versions of dom4j and JUnit 4 contain known CVEs. Upgrading reduces potential vulnerabilities.
• Maintainability: Newer releases are actively maintained and compatible with modern build and IDE tooling.
• Testing Improvements: JUnit 5 provides a modern testing platform, better extensions, and long-term support; aligning the project will make future maintenance easier.
• Future Compatibility: Using a JUnit BOM simplifies future upgrades and dependency alignment.

Fixes #1