Remove reactor-kafka support in 4.0.0 (#3946)

* Remove reactor-kafka based components - ReactiveKafkaConsumerTemplate and
  ReactiveKafkaProducerTemplate - and the associated test classes and packages,
  in 4.0.0 version.
* Remove the optional dependency on reactor-kafka from the build

See this blog for more details:
https://spring.io/blog/2025/05/20/reactor-kafka-discontinued

Signed-off-by: Soby Chacko <[email protected]>

Author: sobychacko

build.gradle

@@ -302,7 +302,6 @@ project ('spring-kafka') {
 		optionalApi "com.jayway.jsonpath:json-path:$jaywayJsonPathVersion"
 
 		optionalApi 'io.projectreactor:reactor-core'
-		optionalApi 'io.projectreactor.kafka:reactor-kafka'
 		optionalApi 'io.micrometer:micrometer-core'
 		optionalApi 'io.micrometer:micrometer-tracing'