Add support for Redmine 5.0.13, 5.0.14, 5.1.9, 5.1.10, 6.0.6, 6.0.7

[Art4]

close #448, close #451

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.63%. Comparing base (da184cc) to head (ac720c5).
⚠️ Report is 5 commits behind head on v2.x.

Additional details and impacted files

@@            Coverage Diff            @@
##               v2.x     #452   +/-   ##
=========================================
  Coverage     98.63%   98.63%           
  Complexity      659      659           
=========================================
  Files            29       29           
  Lines          2056     2056           
=========================================
  Hits           2028     2028           
  Misses           28       28           

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