chore(smoke-tests): use smokeTestApp conventions for spring-boot-2.7-webflux

`gradleVersion` and `javaLauncher` are now provided by the
`dd-trace-java.smoke-test-app` plugin's defaults (Gradle 8.14.5, JDK 21);
drop the explicit overrides from this module's outer build script.

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

Author: bric3

dd-smoke-tests/spring-boot-2.7-webflux/build.gradle

@@ -7,12 +7,6 @@ apply from: "$rootDir/gradle/java.gradle"
 description = 'Spring Boot 2.7 Webflux Smoke Tests.'
 
 smokeTestApp {
-  gradleVersion = "8.14.5"
-  // Nested daemon runs on JDK 21; the inner build keeps `sourceCompatibility = 1.8`
-  // so javac cross-compiles to Java 8 bytecode (Gradle passes `--release 8` automatically).
-  javaLauncher = javaToolchains.launcherFor {
-    languageVersion = JavaLanguageVersion.of(21)
-  }
   application {
     taskName = 'webfluxBuild'
     nestedTasks = ['bootJar']