Start building against Spring Kafka 2.8.6 snapshots

See gh-31018

spring-boot-project/spring-boot-dependencies/build.gradle

@@ -1738,7 +1738,7 @@ bom {
 			]
 		}
 	}
-	library("Spring Kafka", "2.8.5") {
+	library("Spring Kafka", "2.8.6-SNAPSHOT") {
 		prohibit("[3.0.0-M1,)") {
 			because "it uses Spring Framework 6"
 		}