Fixed milestone query for imported members not using correct date

refs https://www.notion.so/ghost/Marketing-Milestone-email-campaigns-1d2c9dee3cfa4029863edb16092ad5c4?pvs=4

- I was passing in the days threshold of 7 to compare with the `created_at` value instead of creating an actual new data to compare

ghost/core/core/server/services/milestones/MilestoneQueries.js

@@ -31,10 +31,13 @@ module.exports = class MilestoneQueries {
      * @returns {Promise<boolean>}
      */
     async hasImportedMembersInPeriod() {
+        const importedThreshold = new Date();
+        importedThreshold.setDate(importedThreshold.getDate() - MIN_DAYS_SINCE_IMPORTED);
+
         const [hasImportedMembers] = await this.#db.knex('members_subscribe_events')
             .count('id as count')
             .where('source', '=', 'import')
-            .where('created_at', '>=', MIN_DAYS_SINCE_IMPORTED);
+            .where('created_at', '>=', importedThreshold);
 
         return hasImportedMembers?.count > 0;
     }