Reduce difficulty retrieval thread count

[smoogipoo]

Comparing release vs master shows three changes: https://github.com/ppy/osu-performance/compare/release?expand=1

• Avoid using mysql 8 "rank" keyword in sql
• Reduce step size
• Reduce difficulty retrieval thread count

The first two are already present in master, created via different commit hashes so the comparison doesn't properly de-dupe them: https://github.com/ppy/osu-performance/blob/d4b8dee15f9f442b83c4f1475881b7e17f84cc74/src/performance/Processor.cpp#L1035-L1037 https://github.com/ppy/osu-performance/blob/d4b8dee15f9f442b83c4f1475881b7e17f84cc74/src/performance/Processor.cpp#L572-L574 But the last isn't: https://github.com/ppy/osu-performance/blob/d4b8dee15f9f442b83c4f1475881b7e17f84cc74/src/performance/Processor.cpp#L67-L68