Upgrade Java 17 to 21 and Spring Boot 3.3.3 to 4.0.0

[devin-ai-integration]

Summary

This PR upgrades the Java and Spring Boot versions to their latest stable releases:

• Spring Boot: 3.3.3 → 4.0.0 (major version upgrade)
• Java: 17 → 21 (LTS upgrade)
• Maven Compiler Plugin: 3.8.0 → 3.13.0 with source/target updated from 1.8 to 21
• Dockerfile: Updated base images to use Eclipse Temurin JDK 21

Review & Testing Checklist for Human

This is a high-risk PR due to the major Spring Boot version upgrade. Please verify:

• Spring Boot 4.0.0 compatibility: Review the Spring Boot 4.0 migration guide for breaking changes. The app uses Spring Security, JPA, and Thymeleaf which may have API changes.
• Docker build verification: Build the Docker image locally to confirm the new base images (maven:3.9-eclipse-temurin-21 and eclipse-temurin:21-jre-alpine) work correctly
• Application startup test: Run the containerized application with MySQL to verify it starts without errors
• Spring Security configuration: Verify SecurityConfig.java is compatible with Spring Security 7 (bundled with Spring Boot 4.0)
• MySQL connector compatibility: Confirm MySQL Connector 8.0.33 works with Java 21

Recommended test plan:

1. Build Docker image: docker build -t bankapp-test .
2. Run with docker-compose: docker-compose up
3. Verify application starts and login/registration flows work

Notes

• Local Maven build was not possible because the dev environment has Java 17 installed. The build will occur in Docker/CI which uses the Java 21 images.
• The previous maven-compiler-plugin was configured for Java 1.8 despite java.version being 17 - this inconsistency has been corrected.

Requested by: Shawn Azman (@ShawnAzman)
Devin session: https://app.devin.ai/sessions/cf83fccd463743b2959d4f40aaff46ec

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