feat: Modernize project for Java 21, MySQL 8, and modern IDEs

[aristorinjuang]

This updates the IDDD samples project (originally targeting Java 7) to work with modern development environments and infrastructure.

Infrastructure Configuration

• Make MySQL and RabbitMQ credentials configurable via environment variables (MYSQL_HOST, MYSQL_PORT, MYSQL_USER, MYSQL_PASSWORD, RABBITMQ_HOST, RABBITMQ_USER, RABBITMQ_PASSWORD)
• Update Spring XML configs to use property placeholders with defaults
• Pass environment variables to test JVM via Gradle test task

Java 21 Compatibility

• Upgrade Gradle wrapper from 2.3 to 8.5
• Set source/target compatibility to Java 21
• Add JVM args for LevelDB internal API access (--add-opens)
• Fix scrollable ResultSet support for MySQL 8.x connector

Dependency Upgrades

• RabbitMQ client: 3.0.4 → 5.18.0 (fix API changes in MessageConsumer, BrokerChannel)
• MySQL connector: 5.1.6 → 8.0.33 (update driver class to com.mysql.cj.jdbc.Driver, add allowPublicKeyRetrieval/useSSL params)
• Spring: 2.5.6 → 3.2.18.RELEASE (maintains Hibernate 3.x compatibility)

IDE Support

• Update .gitignore for IDE-specific files (.settings/, bin/, .metadata/)
• Tested on: Neovim with jdtls, IntelliJ IDEA, Eclipse, VS Code

Documentation

• Update README.md with Java 21 requirement
• Add Test section with environment variables and SQL setup instructions

Test Results

All 341 tests pass with proper infrastructure configuration:

• iddd_agilepm: 106 tests
• iddd_collaboration: 70 tests
• iddd_common: 60 tests
• iddd_identityaccess: 105 tests