Remove CI with GCC 6

[awvwgk]

GCC 6 is not available anymore for MacOS runner

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (fad495b) 65.95% compared to head (eb4244e) 65.95%.

❗ Current head eb4244e differs from pull request most recent head e0173d2. Consider uploading reports for the commit e0173d2 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #144   +/-   ##
=======================================
  Coverage   65.95%   65.95%           
=======================================
  Files          29       29           
  Lines        3169     3169           
  Branches     1350     1350           
=======================================
  Hits         2090     2090           
- Misses        387      393    +6     
+ Partials      692      686    -6     

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