KAFKA-20872: Integrate api-checker checks into CI

[dino2895]

Summary

The api-checker module is a composite-included Gradle build, so its
independent tests and verification tasks are not executed by Kafka's
existing root test task.

This change:

• adds a dedicated api-checker CI job;
• runs the core, gradle-plugins, and maven-plugin check tasks;
• tests the module on Java 17 and Java 25;
• archives its test and verification reports;
• makes CI checks completed depend on the new job.

The dedicated job does not participate in Kafka's normal/new/flaky test
catalog because the included build does not use that test-selection
infrastructure.

JIRA: https://issues.apache.org/jira/browse/KAFKA-20872

Testing

The following checks passed locally on both Java 17 and Java 25:

./gradlew --no-build-cache --rerun-tasks --continue --no-scan \
  :api-checker:core:check \
  :api-checker:gradle-plugins:check \
  :api-checker:maven-plugin:check