update CI to run on 1.10 (LTS) instead of 1.6, which is no longer supported

[ajwheeler]

No description provided.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.73%. Comparing base (ce675e6) to head (65c917c).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #617      +/-   ##
==========================================
+ Coverage   87.25%   87.73%   +0.47%     
==========================================
  Files          29       29              
  Lines        1907     1908       +1     
==========================================
+ Hits         1664     1674      +10     
+ Misses        243      234       -9     

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