Upgrade to Kotlin 1.6.20-M1

This commit upgrades the Kotlin dependencies to 1.6.20-M1 and configures
the build to generate Java 17 bytecode for Kotlin classes.

Closes gh-27814

spring-webflux/src/test/kotlin/org/springframework/web/reactive/result/method/annotation/CoroutinesIntegrationTests.kt

@@ -17,6 +17,7 @@
 package org.springframework.web.reactive.result.method.annotation
 
 import kotlinx.coroutines.Deferred
+import kotlinx.coroutines.DelicateCoroutinesApi
 import kotlinx.coroutines.GlobalScope
 import kotlinx.coroutines.async
 import kotlinx.coroutines.delay
@@ -115,6 +116,7 @@ class CoroutinesIntegrationTests : AbstractRequestMappingIntegrationTests() {
 	@ComponentScan(resourcePattern = "**/CoroutinesIntegrationTests*")
 	open class WebConfig
 
+	@OptIn(DelicateCoroutinesApi::class)
 	@RestController
 	class CoroutinesController {