The query to select the next 5 blogs to upgrade was ordered by `registered`, which isn't indexed. This causes the query to table scan, which will be slow on networks with many blogs.
The query only needs to be ordered by something that won't change, so ordering by `blog_id` is a good replacement. `blog_id` is indexed, and it's the only column being returned, so MySQL is able to optimize for a fast index read.
Props fliespl.
Fixes#37612.
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