Save patchwork armor with right tech level

[pavelbraginskiy]

Saving a mixed-tech unit with patchwork armor often results in nonsense in the MTF file like "Clan Heavy Ferro-Fibrous (Inner Sphere)". This armor fails to load and falls back to Standard Armor.

This PR fixes the code to save the tech level of the armor, not of the mech.

See also MegaMek/megameklab#1634, which fixes a different bug in MML that causes the same problem through an unrelated mistake.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 29.11%. Comparing base (a8d595a) to head (34e042c).
Report is 17 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #6078      +/-   ##
============================================
- Coverage     29.11%   29.11%   -0.01%     
+ Complexity    13978    13977       -1     
============================================
  Files          2601     2601              
  Lines        267075   267092      +17     
  Branches      47743    47749       +6     
============================================
+ Hits          77767    77768       +1     
- Misses       185403   185417      +14     
- Partials       3905     3907       +2     

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