build: upgrade to Java 21

[devin-ai-integration]

Summary

Upgrades the build/runtime to Java 21 (latest LTS, supported by the existing Spring Boot 3.3.3). Previously pom.xml declared java.version 17 but the maven-compiler-plugin was still pinned to Java 8 bytecode, and the Docker images used Java 17 — this aligns everything on 21.

pom.xml:

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

  <artifactId>maven-compiler-plugin</artifactId>
- <version>3.8.0</version>
- <configuration><source>1.8</source><target>1.8</target></configuration>
+ <configuration><release>${java.version}</release></configuration>

Dropping the explicit plugin <version> lets it inherit the version managed by spring-boot-starter-parent, and <release> replaces the deprecated source/target pair (which was incorrectly producing Java 8 bytecode).

Dockerfile:

- FROM maven:3.8.3-openjdk-17 as builder
+ FROM maven:3.9-eclipse-temurin-21-alpine as builder
- FROM openjdk:17-alpine as deployer
+ FROM eclipse-temurin:21-jre-alpine as deployer

Switched from the deprecated openjdk images to maintained Eclipse Temurin 21 images.

Verified with mvn clean install -DskipTests under JDK 21 (build succeeds).

Link to Devin session: https://app.devin.ai/sessions/fe216d1f2a0b469ca32fad8518f016a2

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