[Fluxgate] Support Springboot-2.x with java-11

[rojae]

[Fluxgate] Support Springboot-2.x with java-11

• Add fluxgate-spring-boot2-starter module for Spring Boot 2.7.x (Java 11+)
• Rename fluxgate-spring-boot-starter to fluxgate-spring-boot3-starter
• Add fluxgate-sample-standalone-java11 sample application
• Update CI/CD with multi-JDK testing (Java 11, 17, 21)
• Adjust JaCoCo coverage thresholds per module
• Use javax.servlet for Spring Boot 2.x, jakarta.servlet for 3.x

[Renewal] fluxgate-springboot-starter

<!-- For Spring Boot 3.x (Java 17+) -->
<dependency>
    <groupId>io.github.openfluxgate</groupId>
    <artifactId>fluxgate-spring-boot3-starter</artifactId>
    <version>0.3.2</version>
</dependency>

<!-- For Spring Boot 2.x (Java 11+) -->
<dependency>
    <groupId>io.github.openfluxgate</groupId>
    <artifactId>fluxgate-spring-boot2-starter</artifactId>
    <version>0.3.2</version>
</dependency>

[rojae]

This is implemented in release-0.3.3