updated Project.toml and ci.yml

[KlausC]

use default architecture in CI.yml and use release label 'lts' instead of '1.6'
allow MAT release 0.12 in Project.toml

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.71%. Comparing base (f1e0f23) to head (63d9269).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #140      +/-   ##
==========================================
+ Coverage   92.50%   92.71%   +0.20%     
==========================================
  Files          14       14              
  Lines        2416     2444      +28     
==========================================
+ Hits         2235     2266      +31     
+ Misses        181      178       -3     

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