setting advancement date

rfcs/text/0009-data_stream-fields.md

@@ -2,7 +2,7 @@
 <!-- Leave this ID at 0000. The ECS team will assign a unique, contiguous RFC number upon merging the initial stage of this RFC. -->
 
 - Stage: **2 (draft)** <!-- Update to reflect target stage. See https://elastic.github.io/ecs/stages.html -->
-- Date: **2020-11-11** <!-- The ECS team sets this date at merge time. This is the date of the latest stage advancement. -->
+- Date: **2021-01-04** <!-- The ECS team sets this date at merge time. This is the date of the latest stage advancement. -->
 
 When introducing the new indexing strategy for Elastic Agent which uses data streams, we found that adding a few [constant_keyword](https://www.elastic.co/guide/en/elasticsearch/reference/master/keyword.html#constant-keyword-field-type) fields corresponding to the central components in the new indexing strategy would be advantageous.