Add a test-run with Mill 0.12.14 in a Mill 0.11 project

[lefou]

Download newer Mill versions from Maven Central repository.

The Mill 0.12 itests run only, if the JVM is at least version 11, due to new minimal requirements in Mill 0.12.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.75%. Comparing base (94eeea7) to head (ed08cec).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #220      +/-   ##
==========================================
+ Coverage   89.79%   93.75%   +3.95%     
==========================================
  Files           5        5              
  Lines          49       48       -1     
  Branches        2        2              
==========================================
+ Hits           44       45       +1     
+ Misses          5        3       -2     

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