Improve docs for global default poller

* Fix Kotlin deprecation warning in the test
* Replace wrong `IntervalTrigger` mentioning in the docs to the proper `PeriodicTrigger`
* Fix code snippet in the `channel-adapter.adoc`

spring-integration-core/src/test/kotlin/org/springframework/integration/dsl/KotlinDslTests.kt

@@ -258,7 +258,7 @@ class KotlinDslTests {
 		@Bean
 		fun functionFlow2() =
 			integrationFlow<Function<*, *>> {
-				transform<String> { it.toLowerCase() }
+				transform<String> { it.lowercase() }
 				filter(UnexpiredMessageSelector())
 				route<Message<*>, Any?>({ null }) { defaultOutputToParentFlow() }
 				route<Message<*>> { m -> m.headers.replyChannel }