chore(smoke-tests): remove unused Spring Boot plugin from kafkaschemaregistry

The Spring Boot Gradle plugin was applied but the module never produced a
bootJar/bootWar — it ships a shadow jar instead. Removing the plugin
eliminates a Gradle-9 blocker (the plugin's pre-3.5.0 versions call
`Configuration.getUploadTaskName()`, removed in Gradle 9) with no
behavioural change.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: bric3

dd-smoke-tests/datastreams/kafkaschemaregistry/build.gradle

@@ -3,11 +3,9 @@ import com.github.jengelman.gradle.plugins.shadow.tasks.ShadowJar
 plugins {
   id 'com.gradleup.shadow'
   id 'java'
-  id 'org.springframework.boot' version '2.6.3'
 }
 
 apply from: "$rootDir/gradle/java.gradle"
-apply from: "$rootDir/gradle/spring-boot-plugin.gradle"
 description = 'Kafka Smoke Tests.'
 
 tasks.named("jar", Jar) {