Bug 1788787 - Port bug 1788708: Set default artifact retention to 1 year r=rjl

Differential Revision: https://phabricator.services.mozilla.com/D156240

--HG--
extra : amend_source : 3557b4154bd2284695aabcb5f17a157f7efbcb4a

taskcluster/ci/config.yml

@@ -297,4 +297,4 @@ expiration-policy:
         default:
             default: 3 months
             medium: 1 year
-            long: 3 years
+            long: 1 year