feature: migrate to Java 21 (LTS) and modernize Docker base images

[devin-ai-integration]

Summary

Migrates the BankApp from its inconsistent Java config to Java 21 (LTS). Previously pom.xml had <java.version>17</java.version> but an explicit maven-compiler-plugin overrode it to compile for 1.8. This resolves that conflict and lets the spring-boot-starter-parent (3.3.3) manage compilation.

pom.xml

-<java.version>17</java.version>
+<java.version>21</java.version>

-<plugin>
-  <artifactId>maven-compiler-plugin</artifactId>   <!-- forced source/target 1.8 -->
-  <version>3.8.0</version>
-  <configuration><source>1.8</source><target>1.8</target></configuration>
-</plugin>

-<groupId>mysql</groupId>                  <!-- legacy coordinate -->
-<artifactId>mysql-connector-java</artifactId>
-<version>8.0.33</version>
+<groupId>com.mysql</groupId>              <!-- modern artifact, version managed by parent -->
+<artifactId>mysql-connector-j</artifactId>

Dockerfile (two-stage)

-FROM maven:3.8.3-openjdk-17 as builder      # deprecated openjdk-based maven tag
+FROM maven:3.9-eclipse-temurin-21 as builder
-FROM openjdk:17-alpine as deployer          # deprecated Docker Hub openjdk image
+FROM eclipse-temurin:21-jre-alpine as deployer

CI/CD & Kubernetes — no changes required

• Jenkinsfile, GitOps/Jenkinsfile, and vars/*.groovy (incl. deploy.groovy) build via Docker / docker compose and contain no jdk tool declarations, Java-version agent images, or Maven steps pinned to Java 17 — the JDK is fully controlled by the Dockerfile.
• kubernetes/bankapp-deployment.yml uses the app image tag trainwithshubham/bankapp-eks:v2 (not a Java-versioned base image); mysql:8.0 is unrelated. Nothing to update.

Verification

mvn clean verify with Java 21 (against a live MySQL 8): BUILD SUCCESS, compiles with release 21, tests pass (Tests run: 1, Failures: 0, Errors: 0). No deprecated-API or InaccessibleObjectException regressions surfaced.

Link to Devin session: https://app.devin.ai/sessions/c6aa88af32784ca094fc2d855cfa60ff
Requested by: @amitmanchella-cog

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