updateCollation.php: Switch back to using cl_from index for now
Using the cl_sortkey index instead (to reduce disruption to a live
site), as currently implemented, seems to have two serious problems:
- MySQL / MariaDB filesorts all rows that "sort above the given row [the last row of the previous batch]", not just a single category at a time until the row limit is reached.
- The current approach to pagination is broken in that it does not work with ENUM columns such as cl_type, causing 'file' rows to be skipped, or rows of any type to be repeated. See T119173.
This reverts part of commit a43f751cf6b6849e.