ci: Remove ``os`` from test workflow matrix

[marksweb]

Unused matrix option removed because the workflow always runs on latest ubuntu.

Checks marked as "Expected — Waiting for status to be reported" won't return a response because they're not going to run.

That's GH adding the OS from the matrix to the check name even though we never ran checks against the OS matrix values. I'll need to update the required checks once this is cleaned up.

[codecov[bot]]

Codecov Report

Merging #270 (aa4bdab) into master (d709026) will decrease coverage by 0.37%. The diff coverage is n/a.

:exclamation: Current head aa4bdab differs from pull request most recent head e56888f. Consider uploading reports for the commit e56888f to get more accurate results

@@            Coverage Diff             @@
##           master     #270      +/-   ##
==========================================
- Coverage   87.61%   87.23%   -0.38%     
==========================================
  Files          29       29              
  Lines        1591     1528      -63     
  Branches      260      247      -13     
==========================================
- Hits         1394     1333      -61     
+ Misses        141      140       -1     
+ Partials       56       55       -1     

Impacted Files	Coverage Δ
djangocms_versioning/cms_config.py	83.33% <0.00%> (-1.13%)	:arrow_down:
djangocms_versioning/cms_toolbars.py	95.08% <0.00%> (-0.64%)	:arrow_down:
djangocms_versioning/admin.py	86.60% <0.00%> (-0.40%)	:arrow_down:
djangocms_versioning/datastructures.py	95.18% <0.00%> (-0.38%)	:arrow_down:

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