Bump GPU CI to CUDA 11.8

[charlesbluca]

This addresses the planned dropping of the CUDA 11.5 miniforge-cuda images we currently depend on in rapidsai/miniforge-cuda#55

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (5d8da2c) 85.55% compared to head (7ea221b) 85.55%.
Report is 2 commits behind head on main.

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Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1275   +/-   ##
=======================================
  Coverage   85.55%   85.55%           
=======================================
  Files          77       77           
  Lines        4257     4258    +1     
  Branches      758      758           
=======================================
+ Hits         3642     3643    +1     
  Misses        446      446           
  Partials      169      169           

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