feat: Spring Boot 3.2 compatibility

[burkedavison]

• Verify isReadable=true is correct for SpannerCompositeKeyProperty
• Discuss change in generic types of PubSubSubscriptionHealthIndicatorAutoConfiguration with @meltsufin
• Add Java 21 check

[burkedavison]

Prior to updating samples' pom.xml parent to use org.springframework.boot:spring-boot-starter-parent:3.2.0-RC1:

Caused by: 
  org.springframework.beans.factory.BeanCreationException: 
    Error creating bean with name 'compositeCompatibilityVerifier' defined in class path resource [org/springframework/cloud/configuration/CompatibilityVerifierAutoConfiguration.class]: 
      Failed to instantiate [org.springframework.cloud.configuration.CompositeCompatibilityVerifier]: 
        Factory method 'compositeCompatibilityVerifier' threw exception with message: 
          Spring Cloud/ Spring Boot version compatibility checks have failed: 
            [[VerificationResult@4182a651 
              description = 'Spring Boot [3.1.5] is not compatible with this Spring Cloud release train', 
              action = 'Change Spring Boot version to one of the following versions [3.2.x] .

You can find the latest Spring Boot versions here [https://spring.io/projects/spring-boot#learn]. 
If you want to learn more about the Spring Cloud Release train compatibility, you can visit this page [https://spring.io/projects/spring-cloud#overview] and check the [Release Trains] section.
If you want to disable this check, just set the property [spring.cloud.compatibility-verifier.enabled=false]']]

See Spring Cloud's Release Train compatibility table