Upgrade from Java 17 to Java 21

[devin-ai-integration]

Summary

Upgrades the entire project from Java 17 to Java 21 across build config, container images, and documentation.

Changes

• pom.xml: java.version 17 → 21; maven-compiler-plugin updated to 3.13.0 with source/target 21 (was 1.8)
• Dockerfile: Builder image → maven:3.9-eclipse-temurin-21-alpine; Runtime image → eclipse-temurin:21-jre-alpine
• README.md & README-K8S.md: Jenkins JRE install updated from openjdk-17-jre to openjdk-21-jre
• Jenkinsfile: No changes needed — pipeline uses shared library functions with no direct Java version references
• docker-compose.yml: No changes needed — no Java version references present

Build verified locally with mvn clean install -DskipTests using Java 21.

Review & Testing Checklist for Human

• Verify mvn clean install -DskipTests succeeds with JDK 21
• Verify docker build -t bankapp:java21 . builds successfully
• Confirm Jenkins shared library stages work with the updated Docker images

Notes

• The maven-compiler-plugin was previously at version 3.8.0 with source/target 1.8, which was inconsistent with the java.version property of 17. Both have been aligned to 21.
• The Dockerfile now uses Eclipse Temurin (Adoptium) images instead of the deprecated openjdk images.

Link to Devin session: https://app.devin.ai/sessions/cf5ce8a1ee674c319f28c1de91efe73d
Requested by: @joao-cognition

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