ci: Drop ubuntu-16.04

[peaceiris]

No description provided.

[codecov]

Codecov Report

Merging #606 (e5bbb3f) into main (a43024e) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #606   +/-   ##
=======================================
  Coverage   61.73%   61.73%           
=======================================
  Files           4        4           
  Lines         277      277           
  Branches       49       49           
=======================================
  Hits          171      171           
  Misses        105      105           
  Partials        1        1           

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