fix: Added correct relationship description to get_created_by admin_order_field

[adam-murray]

Description

• In admin menus which make use of the ExtendedVersionAdminMixin, the created_by field was not ordering properly, this is due to the entire relationship not being described in the admin_order_field configuration. By ensuring the last field in the target is set to username, rather than simply the User model, we can sort alphabetically.

Checklist

• [x] I have opened this pull request against master
• [x] I have added or modified the tests when changing logic
• [x] I have followed the conventional commits guidelines to add meaningful information into the changelog
• [x] I have read the contribution guidelines and I have joined #workgroup-pr-review on Slack to find a “pr review buddy” who is going to review my pull request.

[codecov[bot]]

Codecov Report

Merging #280 (0f45bff) into master (c41e4f2) will increase coverage by 0.18%. The diff coverage is 96.00%.

@@            Coverage Diff             @@
##           master     #280      +/-   ##
==========================================
+ Coverage   87.43%   87.61%   +0.18%     
==========================================
  Files          29       29              
  Lines        1568     1591      +23     
  Branches      256      260       +4     
==========================================
+ Hits         1371     1394      +23     
  Misses        141      141              
  Partials       56       56              

Impacted Files	Coverage Δ
djangocms_versioning/cms_toolbars.py	95.71% <94.44%> (+0.63%)	:arrow_up:
djangocms_versioning/__init__.py	100.00% <100.00%> (ø)
djangocms_versioning/admin.py	86.99% <100.00%> (+0.14%)	:arrow_up:

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