viewedAt field is now a datetime string only [SA-8302]

Author: armatronic

openapi/paths/news@lists@feed.yaml

@@ -46,7 +46,7 @@ get:
                             The datetime the news article was originally read by the user.
 
                             May be null, in which case the user has not read the article
-                        - $ref: ../components/schemas/dateTime.yaml
+                        - $ref: ../components/schemas/dateTimeString.yaml
                     _links:
                       $ref: ../components/schemas/news/links.yaml
     default: