Fully-qualify KafkaContainer to make it clear which one we support

Testcontainers now has org.testcontainers.containers.KafkaContainer
(that we support) and org.testcontainers.kafka.KafkaContainer (that
we don't yet support). This commit fully-qualifies the name so that
it's clear which KafkaContainer we're referring to.

Closes gh-40699

Author: wilkinsona

Diff for: spring-boot-project/spring-boot-docs/src/docs/asciidoc/features/testing.adoc

@@ -999,7 +999,7 @@ The following service connection factories are provided in the `spring-boot-test
 | Containers of type `JdbcDatabaseContainer`
 
 | `KafkaConnectionDetails`
-| Containers of type `KafkaContainer` or `RedpandaContainer`
+| Containers of type `org.testcontainers.containers.KafkaContainer` or `RedpandaContainer`
 
 | `LiquibaseConnectionDetails`
 | Containers of type `JdbcDatabaseContainer`