Update xunit to v3

[phnx47]

• Update xunit to v3 for non-net462 targets, keeping v2 for net462
• Adapt PushgatewayFixture with #if NET462 directives for IAsyncLifetime signature changes (Task → ValueTask)
• Add net10.0 target framework for tests

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.83%. Comparing base (bc8117e) to head (ad27375).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #184   +/-   ##
=======================================
  Coverage   91.83%   91.83%           
=======================================
  Files           3        3           
  Lines          98       98           
  Branches       16       16           
=======================================
  Hits           90       90           
  Misses          3        3           
  Partials        5        5           

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