[core] Adjust desynth success penalty for -1 to -7 range to 5.0f per difficulty

[Hatberg]

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• I have read and understood the Contributing Guide and the Code of Conduct.
• I have tested my code and the things my code has changed since the last commit in the PR and will test after any later commits.

What does this pull request do?

The Desynthesis successRate penalty was off by two digits in the -1 to -7 range. Changed the penalty per level from 0.05f to 5.0f.

Steps to test these changes

Add print logging to synthutils at the -1/-7 desynthesis range (lines 605, 620) and rebuild

!additem 4100 20 (lightning crystals)
!additem 596 20 (quadav backplates)

Desynth items in game, after setting smithing levels:
!setskill smithing 51 (through 41)
Read out the console logs in map-server

Before:

After:

Sources: https://www.bluegartr.com/threads/135055-Extensive-Desynthesis-Rate-Research