feat: Migrate from Java 17 to Java 21

[devin-ai-integration]

Summary

Migrates the project from Java 17 to Java 21 across build config, Docker images, and documentation. Also rewrites README.md for improved readability (proper markdown headings, table of contents, tech stack table, removal of raw HTML).

Key changes:

• pom.xml: java.version 17 → 21; removed maven-compiler-plugin that was forcing Java 1.8 source/target
• Dockerfile: Switched to Eclipse Temurin-based images (maven:3.9.6-eclipse-temurin-21, eclipse-temurin:21-jre-alpine) since official OpenJDK Docker images don't publish Java 21 Alpine variants
• README.md: openjdk-17-jre → openjdk-21-jre in Jenkins install; full rewrite replacing HTML <b>/<mark> tags with standard markdown, adding ToC and structured sections

Spring Boot version intentionally kept at 3.3.3 (already supports Java 21 since 3.2.0+).

Review & Testing Checklist for Human

• Verify Docker image tags exist: Confirm maven:3.9.6-eclipse-temurin-21 and eclipse-temurin:21-jre-alpine are valid tags on Docker Hub. The original task suggested maven:3.8.3-openjdk-21 and openjdk:21-alpine, but those don't exist — the replacements chosen here need to be validated.
• Run a Docker build (docker build .) to confirm both stages resolve and the app compiles successfully under Java 21
• Spot-check the README rewrite — the diff is large (~575 lines rewritten). Verify no sections, image links, or commands were accidentally dropped compared to the original
• Confirm the maven-compiler-plugin removal doesn't affect any other build behavior (Spring Boot parent should handle compiler settings via java.version property)

Notes

• The added XML comment (<!-- Spring Boot 3.2.0+ supports Java 21 -->) on the version line is informational but could be considered noise — remove if unwanted.
• No local build or test was performed; changes are config/docs only but Docker image tag correctness is critical.

Requested by: @joao-cognition
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[devin-ai-integration]

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