Bump markdown to ~2.1.6

Change-Id: I93392eb3e8cf5a65a8c85b11a4ed8d8014377f0d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/155842
Reviewed-by: Nate Bosch <[email protected]>

Author: srawlins

DEPS

@@ -116,7 +116,7 @@ vars = {
   "linter_tag": "0.1.117",
   "logging_rev": "9561ba016ae607747ae69b846c0e10958ca58ed4",
   "markupsafe_rev": "8f45f5cfa0009d2a70589bcda0349b8cb2b72783",
-  "markdown_rev": "dd150bb64c5f3b41d31f20f399ae2a855f7f8c00",
+  "markdown_rev": "acaddfe74217f62498b5cf0cf5429efa6a700be3",
   "matcher_rev": "9cae8faa7868bf3a88a7ba45eb0bd128e66ac515",
   "mime_rev": "179b5e6a88f4b63f36dc1b8fcbc1e83e5e0cd3a7",
   "mockito_rev": "d39ac507483b9891165e422ec98d9fb480037c8b",