gradle: bump org.junit:junit-bom in /client-samples/java/websockets

Bumps [org.junit:junit-bom](https://github.com/junit-team/junit-framework) from 5.13.4 to 6.0.3.
- [Release notes](https://github.com/junit-team/junit-framework/releases)
- [Commits](junit-team/junit-framework@r5.13.4...r6.0.3)

---
updated-dependencies:
- dependency-name: org.junit:junit-bom
  dependency-version: 6.0.3
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

Author: dependabot[bot]

client-samples/java/websockets/build.gradle.kts

@@ -25,7 +25,7 @@ dependencies {
     // testing
     testImplementation(group="com.squareup.okhttp3", name="mockwebserver", version="5.3.2")
     testImplementation(group="io.fabric8", name="mockwebserver", version="7.5.2")
-    testImplementation(platform("org.junit:junit-bom:5.13.4"))
+    testImplementation(platform("org.junit:junit-bom:6.0.3"))
     testImplementation(group="org.junit.jupiter", name="junit-jupiter")
     testRuntimeOnly(group="org.junit.platform", name="junit-platform-launcher")
     testImplementation(group="org.mockito", name="mockito-core", version="5.21.0")