feat: add java-21-sample with virtual threads and SSL health indicator

[dingmeng-xue]

Summary

Adds a new java-21-sample module to showcase Java 21 LTS features on Azure Spring Apps, modelled after the existing java-17-sample.

What's Changed

New module: java-21-sample

Stack

• Java 21, Spring Boot 3.4.1, Spring Cloud 2024.0.0
• maven-compiler-plugin 3.13.0 and maven-resources-plugin 3.3.1 pinned in <pluginManagement> to suppress plugin-validation warnings

Java 21 language features demonstrated

• Virtual threads — spring.threads.virtual.enabled=true enables Project Loom for improved concurrency
• Records — Greeting record used as an immutable data carrier in HelloController
• Switch expressions — LTS tier detection in GET /java-version
• Text blocks — response body in GET /java-version
• Pattern-matching instanceof — used in SslController when iterating keystore entries

SSL Health Indicator (Spring Boot 3.4)

• SSL bundle demo configured via spring.ssl.bundle.jks.* with a self-signed JKS keystore
• management.health.ssl.enabled=true activates SslHealthIndicator
• management.health.ssl.certificate-validity-warning-threshold=P30D raises WARNING when the cert expires within 30 days
• GET /actuator/health/ssl — actuator-native health status (UP / WARNING / DOWN)
• GET /ssl/info — human-readable HTML table of certificate metadata (alias, subject, issuer, validity dates, serial)

Deployment

• Standard Plan: --runtime-version Java_21
• Enterprise Plan: --source-path . --build-env BP_JVM_VERSION=21.*

Files Added

File	Purpose
pom.xml	Project build, Java 21 compiler, Spring Boot 3.4.1 / Spring Cloud 2024.0.0
Java21SampleApplication.java	Spring Boot entry point
HelloController.java	Greeting, java-version, and memory endpoints
EurekaController.java	Eureka service registry inspection
SslController.java	SSL bundle certificate metadata endpoint
application.properties	SSL bundle config, health indicator settings, virtual threads
ssl/demo.jks	Self-signed demo keystore (RSA-2048, 365-day validity)
README.md	Overview, endpoint table, local-dev and deployment instructions