Migrate from Java 17 to Java 21

[devin-ai-integration]

Summary

Upgrades the build/runtime target from Java 17 to Java 21. The Spring Boot parent (3.3.3) already supports 21, so no dependency changes were needed.

pom.xml

• <java.version>17</java.version> → 21
• Fixed the maven-compiler-plugin: it had a stale <source>1.8</source>/<target>1.8</target> (and a hardcoded <version>3.8.0</version>). Both the explicit source/target and the version are now removed so the plugin inherits the version from the Spring Boot parent and the release from java.version (compiler now runs with release 21).

Dockerfile

• Builder: maven:3.8.3-openjdk-17 → maven:3.9-eclipse-temurin-21
• Deployer: openjdk:17-alpine → eclipse-temurin:21-jre-alpine (the openjdk images are deprecated and have no 21-alpine tag)

Verification

• ./mvnw clean install with JDK 21: BUILD SUCCESS, compiled with release 21, Tests run: 1, Failures: 0, Errors: 0 (against a live MySQL).
• docker build .: both stages build successfully on the new base images.

Kubernetes manifests (kubernetes/) and Helm charts (helm/bankapp/) reference the container image, not a Java version, and contain no hardcoded JDK base-image tags — left unchanged.

Link to Devin session: https://app.devin.ai/sessions/ca7656e2aa9740fabbff35d379ec17ed
Requested by: @joao-cognition

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