chore(MBA-1550): upgrade Java from 17 to 21

[devin-ai-integration]

Summary

MBA-1550: move the app to JDK 21. Scope is the JDK bump plus proof it still compiles — Spring Boot stays at 3.3.3 (already Java 21 compatible).

pom.xml also had a latent bug: the explicit maven-compiler-plugin config pinned source/target to 1.8, silently overriding java.version — so the bytecode was Java 8 regardless of the property. Replaced with release, driven by the property, and dropped the pinned plugin version so the Spring Boot parent manages it:

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

Dockerfile base images move to Eclipse Temurin (maven:3.9-eclipse-temurin-21, eclipse-temurin:21-jre-alpine) since the openjdk images are deprecated and have no 21-alpine tag; both tags verified to exist upstream.

Validated on JDK 21.0.11: ./mvnw clean compile compiles with [debug parameters release 21], and ./mvnw clean package passes (1 test, against a local MySQL 8). Docker build not exercised.

README*.md mentions of openjdk-17-jre are Jenkins host setup, not the app runtime — left alone.

Devin-Org: engineering

Link to Devin session: https://app.devin.ai/sessions/c3085cdd6d83416a8beba7a8aa8bd471
Requested by: @clivingston-cognition

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Note

Devin errored when opening this Pull Request as clivingston-cognition.
As a fallback, Devin opened this PR as itself.

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