add missing BC for version 6.8 (#1173)

Author: ramunasd

BREAKING_CHANGES.md

@@ -19,6 +19,10 @@
 - Added type hints and return type declarations where possible
   [#897](https://github.com/elastic/elasticsearch-php/pull/897)
 
+# 6.8
+
+- Method taskList() renamed to list() in TasksNamespace
+
 # 6.7
 
 - `{type}` part in `indices.put_mapping` API is not required anymore, see new specification [here](https://github.com/elastic/elasticsearch/blob/v6.7.0/rest-api-spec/src/main/resources/rest-api-spec/api/indices.put_mapping.json)