fix(kafka): [Queue Instrumentation 26] Mark producer interceptor experimental

The raw kafka producer path requires customers to reference
SentryKafkaProducerInterceptor directly by class name, so it should not be
marked internal. Align it with the customer-facing queue tracing surface by
marking it experimental instead.

Audit the remaining Kafka classes still marked internal and keep them as-is:
the Spring bean post processors and Spring record interceptor remain
framework wiring internals rather than direct customer entry points.

Co-Authored-By: Claude <noreply@anthropic.com>

Author: adinauer

sentry-kafka/src/main/java/io/sentry/kafka/SentryKafkaProducerInterceptor.java

@@ -25,7 +25,7 @@
 import org.jetbrains.annotations.NotNull;
 import org.jetbrains.annotations.Nullable;
 
-@ApiStatus.Internal
+@ApiStatus.Experimental
 public final class SentryKafkaProducerInterceptor<K, V> implements ProducerInterceptor<K, V> {
 
   public static final @NotNull String TRACE_ORIGIN = "auto.queue.kafka.producer";