Fix code coverage collection on net481

[tillig]

Code coverage has not been collected in this repository since coverlet.collector moved past 6.x. Same defect and same fix as autofac/Autofac.Multitenant.Wcf#12.

Proposed Changes

• Pin coverlet.collector to [6.0.4], the last release shipping netstandard2.0 build assets. Every release from 8.0.0 on ships net8.0 and later only, which a net481 test project cannot consume, so NuGet resolved the package with no build assets at all — coverlet.collector.dll never reached the test output and the XPlat Code Coverage data collector did not exist.
• Record the constraint at the reference, using exact-version range syntax so a bump has to be deliberate rather than silently dropping coverage again.

Notes

Two things kept this quiet. NuGet does not warn when a package's build assets are framework-incompatible. And the shared ci.yml leaves disable_search off, so codecov logged not_found_files for artifacts/logs/*/coverage.cobertura.xml, searched, found one unrelated file, and uploaded that instead of failing — despite fail_ci_if_error: true. Setting disable_search: true in autofac/.github would turn that into a hard failure across every repo; that is not part of this PR.

Verified locally: restore resolves build assets for net481, the test run produces artifacts/logs/*/coverage.cobertura.xml, and 61 passed / 11 skipped is unchanged (the skips are the Windows-only hosting tests).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.42%. Comparing base (0b3b865) to head (d60f2c4).

Additional details and impacted files

@@             Coverage Diff              @@
##           develop      #44       +/-   ##
============================================
+ Coverage         0   80.42%   +80.42%     
============================================
  Files            0       10       +10     
  Lines            0      235      +235     
  Branches         0       67       +67     
============================================
+ Hits             0      189      +189     
- Misses           0       23       +23     
- Partials         0       23       +23     

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