fix(spring): Use Spring Boot 4 test starter in Spring 7 sample

The Spring 7 sample used libs.springboot.starter.test which resolved
to Spring Boot 2.7.18, causing a classpath conflict with the Spring
Framework 7.x runtime dependencies from the Spring Boot 4 BOM. This
mismatch triggered Kotlin ClasspathEntrySnapshotTransform failures in
the Spring Boot 4.x CI matrix for older versions (4.0.0, 4.0.5).

Co-Authored-By: Claude <noreply@anthropic.com>

Author: adinauer

sentry-samples/sentry-samples-spring-7/build.gradle.kts

@@ -49,7 +49,7 @@ dependencies {
 
   testImplementation(projects.sentrySystemTestSupport)
   testImplementation(libs.kotlin.test.junit)
-  testImplementation(libs.springboot.starter.test) {
+  testImplementation(libs.springboot4.starter.test) {
     exclude(group = "org.junit.vintage", module = "junit-vintage-engine")
   }
 }