build: migrate from Java 17 to Java 21

[devin-ai-integration]

Summary

Migrates the build/runtime toolchain from Java 17 to Java 21. Spring Boot 3.3.3 already supports 21, so no parent bump was needed.

• pom.xml: java.version 17 → 21. Removed the maven-compiler-plugin <configuration> block entirely — it was pinning <source>/<target> to 1.8, conflicting with java.version. The plugin now inherits release=21 from the Spring Boot parent.
• Dockerfile: builder maven:3.8.3-openjdk-17 → maven:3.9-eclipse-temurin-21-alpine; deployer openjdk:17-alpine → eclipse-temurin:21-jre-alpine (the deprecated openjdk images have no 21 tags; Temurin is the maintained replacement).
• README.md / README-K8S.md: openjdk-17-jre → openjdk-21-jre in the Jenkins host setup step.

Verification

• mvn clean install (Java 21) — succeeds.
• docker build -t bankapp:java21 . — succeeds (builder compiles + runtime image builds).

Link to Devin session: https://app.devin.ai/sessions/e3693e722c454f90825b5c18d13969de
Requested by: @joao-cognition

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