feat(api): add project milestone promotion

[StingRayZA]

Adds promotion to Project Milestones

Fixes #1654

[codecov-commenter]

Codecov Report

Merging #1655 (f068520) into main (0b53c0a) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1655      +/-   ##
==========================================
+ Coverage   91.95%   91.97%   +0.02%     
==========================================
  Files          74       74              
  Lines        4323     4324       +1     
==========================================
+ Hits         3975     3977       +2     
+ Misses        348      347       -1     

Flag	Coverage Δ
cli_func_v4	81.86% <100.00%> (+<0.01%)	⬆️
py_func_v4	81.10% <100.00%> (+0.02%)	⬆️
unit	83.67% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
gitlab/v4/objects/milestones.py	100.00% <100.00%> (ø)
gitlab/mixins.py	91.54% <0.00%> (+0.24%)	⬆️

[nejch]

Thanks @StingRayZA looks good, and the PromoteMixin does make sense now.

A group project fixture (instead of personal ones) might be useful in the future if we do more of this.