scenario-tests should use the same xunit version as other repositories that use Arcade

Right now the xunit version in this repo is hardcoded: https://github.com/dotnet/scenario-tests/blob/ddccfec3ccd631fb8341c8b6e4e422e8cb339aa5/eng/Versions.props#L8

Arcade already promotes (and keeps that version up-to-date) that should be used. For source-build, xunit is part of SBE and when updating xunit in Arcade, updating it in SBE is part of that process: https://github.com/dotnet/arcade/blob/main/Documentation/update-xunit.md