Upgrade from Java 17 to Java 21

[devin-ai-integration]

Summary

Upgrades the Spring Boot BankApp from Java 17 to Java 21. This includes updates to:

• pom.xml: Updated java.version property to 21, upgraded maven-compiler-plugin to 3.11.0 with source/target 21 (previously was misconfigured with source/target 1.8)
• Dockerfile: Updated build stage to maven:3.9-eclipse-temurin-21 and runtime stage to eclipse-temurin:21-jre-alpine (using Eclipse Temurin instead of openjdk images per best practices)

The application builds successfully with Java 21. Spring Boot 3.3.3 fully supports Java 21.

Review & Testing Checklist for Human

• Verify the Docker images (maven:3.9-eclipse-temurin-21 and eclipse-temurin:21-jre-alpine) exist and pull correctly
• Build and run the Docker container to confirm the application starts successfully with Java 21
• Test critical application functionality (login, transactions, etc.) in a staging environment with the MySQL database

Note: Local tests could not run due to missing MySQL database infrastructure, so runtime verification is important.

Notes

• Link to Devin run: https://app.devin.ai/sessions/ab07a72e6bca412fa1db7827c61f37bb
• Requested by: Joao Esteves (@joao-cognition)

[devin-ai-integration]

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