Fix a warning in Kotlin tests

spring-messaging/src/test/kotlin/org/springframework/messaging/rsocket/RSocketRequesterExtensionsTests.kt

@@ -18,6 +18,7 @@ package org.springframework.messaging.rsocket
 
 import io.mockk.every
 import io.mockk.mockk
+import kotlinx.coroutines.ExperimentalCoroutinesApi
 import kotlinx.coroutines.flow.Flow
 import kotlinx.coroutines.flow.toList
 import kotlinx.coroutines.runBlocking
@@ -125,6 +126,7 @@ class RSocketRequesterExtensionsTests {
 	}
 
 	@Test
+	@ExperimentalCoroutinesApi
 	fun retrieveFlow() {
 		val responseSpec = mockk<RSocketRequester.ResponseSpec>()
 		every { responseSpec.retrieveFlux(match<ParameterizedTypeReference<*>>(stringTypeRefMatcher)) } returns Flux.just("foo", "bar")