Reset linter warnings with longer decay times

refs https://github.com/TryGhost/Ghost/issues/14101

- we have ~1000 linter warnings from historic code that is awaiting transforms as part of the Ember Octane migration
- the warning delay time of 30 days is too short, we keep hitting it because work on the migration has stalled
- keeping the linting errors in place is useful because the rules are useful for new code and the todo file helps track migration progress
- increased the ignore->warn decay time to 120 days, with a warn->fail time of 60 days after that giving us some time to clear up any remaining issues or add them back to the ignore list before builds start failing

ghost/admin/.lint-todorc.js

@@ -0,0 +1,8 @@
+module.exports = {
+    'ember-template-lint': {
+      daysToDecay: {
+        warn: 120,
+        error: 180,
+      }
+    }
+  };