build: migrate from Java 17 to Java 21

[devin-ai-integration]

Summary

Moves the build and runtime to Java 21 (Spring Boot 3.3.3 already supports it, so the parent stays put).

• pom.xml: java.version 17 → 21, and the pinned maven-compiler-plugin 3.8.0 block with <source>/<target>1.8</source> is dropped so the plugin inherits the parent's managed version and java.version (that 1.8 setting was inconsistent with Spring Boot 3.x, which requires 17+).
• Dockerfile: builder maven:3.8.3-openjdk-17 → maven:3.9-eclipse-temurin-21; runtime openjdk:17-alpine → eclipse-temurin:21-jre-alpine (no openjdk:21-alpine tag exists — those images are deprecated).
• READMEs: Jenkins prerequisite openjdk-17-jre → openjdk-21-jre.

Verified: mvn clean install -DskipTests succeeds and emits class file major version 65 (Java 21); docker build succeeds and the container reports Starting BankappApplication ... using Java 21.0.12 (it then fails on the DB connection, expected with no MySQL running).

Note: this sandbox gets HTTP 429 from Maven Central, so both builds were run through a Central mirror; that's an environment limitation, unrelated to these changes.

Link to Devin session: https://app.devin.ai/sessions/f9d9a829406b4c5cba392415c5c7bb9b
Open in Devin Desktop: https://app.devin.ai/desktop/session/f9d9a829406b4c5cba392415c5c7bb9b?variant=devin
Requested by: @joao-cognition

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✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.