MBA-484: Upgrade Java version to 21 and remove conflicting compiler plugin

[devin-ai-integration]

MBA-484: Upgrade Java version to 21 and remove conflicting compiler plugin

Summary

This PR upgrades the project's Java version from 17 to 21 as specified in JIRA ticket MBA-484. The changes resolve a conflicting maven-compiler-plugin configuration that was targeting Java 1.8 while the project declared Java 17.

Changes made:

• Updated <java.version> property from 17 to 21 in pom.xml (line 30)
• Removed the entire maven-compiler-plugin configuration block (lines 78-86) that was targeting Java 1.8
• Spring Boot 3.3.3 parent POM will now manage compiler settings appropriately for Java 21

Scope: This is a deliberately narrow upgrade focused solely on the Java version configuration. Per task requirements, Docker files, README documentation, MySQL connector, and Kubernetes/Helm configs are intentionally NOT updated in this PR and will be handled in separate tickets.

Review & Testing Checklist for Human

⚠️ Important: Local build verification was not possible due to the dev environment having Java 17 instead of Java 21. The build failed locally with "error: release version 21 not supported". All verification must happen via CI and manual testing.

• Verify CI passes with Java 21 - Confirm the CI environment has Java 21 installed and mvn clean install succeeds
• Manual build test - Clone the branch and verify mvn clean install completes successfully in a Java 21 environment
• Runtime compatibility check - Test starting the application and verify all Spring Boot features work correctly with Java 21
• Dependency compatibility - Review that all dependencies (especially mysql-connector-java 8.0.33) are compatible with Java 21
• Consider deployment config alignment - Note that Dockerfile and K8s configs still reference older Java versions (out of scope for this PR but may need follow-up)

Recommended Test Plan

1. Run mvn clean install in a Java 21 environment
2. Start the application locally or in a test environment
3. Verify database connectivity works
4. Test core banking application features
5. Check application logs for any Java 21-related warnings or errors

Notes

• JIRA Ticket: MBA-484
• Link to Devin run: https://app.devin.ai/sessions/89743c7a03e2486697cd63e4feaa4e50
• Requested by: Jia Wu (@jia-cog)
• Environment Issue: Dev environment reported to have Java 17 instead of Java 21, preventing local compilation testing
• Next Steps: After this PR merges, separate tickets should address Docker image updates, README documentation, and deployment configuration alignment with Java 21

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