Correct a Dashboard status constant in a migration

Summary:
Fixes T9500. All the code is fine in D13836, but the value of the constant got updated (from "open" to "active") and the migration still used the old value.

Correct any affected dashboards to use the proper constant.

This only affected old dashboards: newly created ones use the right constant.

Test Plan: Ran migration, verified that all active dashboards appeared on "Active Dashboards".

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9500

Differential Revision: https://secure.phabricator.com/D14223