Updated tk-core to v0.21.0

Release notes: https://github.com/shotgunsoftware/tk-core/wiki/Release-Notes#v0210

Author: toolkitty

core/core_api.yml

@@ -14,4 +14,4 @@
 location:
   type: app_store
   name: tk-core
-  version: v0.20.35
+  version: v0.21.0