Add Redmine 6.1 and Drop Redmine 5.0 from test matrix

[haru]

No description provided.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.87%. Comparing base (2723a95) to head (4609ac3).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop      #41   +/-   ##
========================================
  Coverage    88.87%   88.87%           
========================================
  Files           16       16           
  Lines          935      935           
========================================
  Hits           831      831           
  Misses         104      104           

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