Bug 1812106 - don't expire merge-automation tasks after 3 months. r=releng-reviewers,bhearsum DONTBUILD

These tasks run basically once a month, so having them expire after 3
months means we're left with very little history to try and investigate
trends/issues.

Differential Revision: https://phabricator.services.mozilla.com/D168280

taskcluster/ci/merge-automation/kind.yml

@@ -69,3 +69,4 @@ jobs:
                         }]
                     default: null
         run-on-projects: []
+        expiration-policy: long