dep: Update to GSL 2.8

[inkydragon]

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 6.87%. Comparing base (ee43384) to head (579f14c).

Additional details and impacted files

@@          Coverage Diff           @@
##           master    #142   +/-   ##
======================================
  Coverage    6.87%   6.87%           
======================================
  Files         189     189           
  Lines        9930    9930           
======================================
  Hits          683     683           
  Misses       9247    9247           

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