[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 +435,14 @@ 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 bug
+                                        // addressed by the FLIP-471, by returning SystemClock,
+                                        // which would reproduce the pre-FLIP-471 behavior (without
+                                        // fixing the underlying bug).
                                         deserializedWatermarkStrategy.createWatermarkGenerator(
                                                 () -> consumerMetricGroup),
                                         immediateOutput,