[FLINK-35886] Leave a note for future Flink 2.0 upgrade

Fixing FLIP-471 in the old source would require some extra work, that we hopefully can avoid by removing the old source before doing the upgrade.

Author: pnowojski

flink-connector-kafka/src/main/java/org/apache/flink/streaming/connectors/kafka/internals/AbstractFetcher.java

@@ -435,6 +435,13 @@ private List<KafkaTopicPartitionState<T, KPH>> createPartitionStateHolders(
                                         kafkaHandle,
                                         deserializedWatermarkStrategy.createTimestampAssigner(
                                                 () -> consumerMetricGroup),
+                                        // When upgrading to Flink 2.0, context has to provide also
+                                        // the input activity clock. This is not trivial for the old
+                                        // sources. Ideally we should drop this old source before
+                                        // this connector is upgraded to Flink 2.0. Otherwise, we
+                                        // can avoid the compilation error without fixing the
+                                        // FLIP-471 bug, by returning SystemClock as the input
+                                        // activity clock. Note that would be incorrect thing to do.
                                         deserializedWatermarkStrategy.createWatermarkGenerator(
                                                 () -> consumerMetricGroup),
                                         immediateOutput,