Treat Kotlin warnings as errors

Closes gh-23566

build.gradle

@@ -86,6 +86,7 @@ configure(allprojects.findAll { (it.name != "framework-bom") } ) { project ->
 			kotlinOptions {
 				jvmTarget = "1.8"
 				freeCompilerArgs = ["-Xjsr305=strict"]
+				allWarningsAsErrors = true
 			}
 		}
 		compileTestKotlin {

spring-messaging/src/main/kotlin/org/springframework/messaging/rsocket/RSocketRequesterExtensions.kt

@@ -17,6 +17,7 @@
 package org.springframework.messaging.rsocket
 
 import io.rsocket.transport.ClientTransport
+import kotlinx.coroutines.ExperimentalCoroutinesApi
 import kotlinx.coroutines.flow.Flow
 import kotlinx.coroutines.reactive.awaitFirstOrNull
 import kotlinx.coroutines.reactive.awaitSingle
@@ -118,6 +119,7 @@ suspend inline fun <reified T : Any> RSocketRequester.ResponseSpec.retrieveAndAw
  * @author Sebastien Deleuze
  * @since 5.2
  */
+@ExperimentalCoroutinesApi
 inline fun <reified T : Any> RSocketRequester.ResponseSpec.retrieveFlow(): Flow<T> =
 		retrieveFlux(object : ParameterizedTypeReference<T>() {}).asFlow()