Added updated_at index to the posts table

ref https://linear.app/tryghost/issue/ONC-82
- added migration for adding a posts.updated_at index

The default order by clause is published_at desc. It makes sense that users may want to return the latest updated posts instead of the last published. Currently we don't have an index for this and it makes for slow data fetching.

ghost/core/core/server/data/migrations/versions/5.85/2024-06-10-14-53-31-add-posts-updated-at-index.js

@@ -0,0 +1,4 @@
+// For information on writing migrations, see https://www.notion.so/ghost/Database-migrations-eb5b78c435d741d2b34a582d57c24253
+const {createAddIndexMigration} = require('../../utils');
+
+module.exports = createAddIndexMigration('posts',['updated_at']);