Bump assertj-core from 3.20.2 to 3.21.0 in /examples

Bumps [assertj-core](https://github.com/assertj/assertj-core) from 3.20.2 to 3.21.0.
- [Release notes](https://github.com/assertj/assertj-core/releases)
- [Commits](assertj/assertj@assertj-core-3.20.2...assertj-core-3.21.0)

---
updated-dependencies:
- dependency-name: org.assertj:assertj-core
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

examples/kafka-cluster/build.gradle

@@ -11,7 +11,7 @@ dependencies {
     testAnnotationProcessor "org.projectlombok:lombok:1.18.10"
     testImplementation 'org.testcontainers:kafka'
     testImplementation 'org.apache.kafka:kafka-clients:3.0.0'
-    testImplementation 'org.assertj:assertj-core:3.20.2'
+    testImplementation 'org.assertj:assertj-core:3.21.0'
     testImplementation 'com.google.guava:guava:23.0'
     testImplementation 'ch.qos.logback:logback-classic:1.2.6'
 }