WL#13600: Deprecate system variable max_length_for_sort_data

Affects: Server-8.0   —   Status: Complete

After fixing BUG#97154 (SORT SHORT BLOBS AS ADDON FIELDS), nothing will be using 
max_length_for_sort_data anymore, so we need to deprecate it.

Currently, max_length_for_sort_data (default 4096) governs one and only one 
thing: If the maximum size of addon fields exceeds 4 kB, we will fall back to 
sorting row IDs (which is expensive, as we need to make a lot of random accesses 
to the table after the sort to actually get the data). However, the typical case 
for this is a JSON blob or geometry column; while they can theoretically be very 
large, yielding a huge maximum length, in practice they will be fairly small, and 
we get poor performance as a result.

This is not to be confused with max_sort_length, which governs at which point we 
will start truncating sort _keys_ for blobs and strings with PAD SPACE 
F1. When trying to set the variable max_length_for_sort_data, one should get a 
deprecation warning.