Bump xunit from 2.4.0 to 2.7.0

[snakefoot]

Bumps xunit from 2.4.0 to 2.7.0.

Commits

• d806fa1 v2.7.0
• 13bb60b #2872: Expand special handling for sets in Assert.Contains/DoesNotContain (v2)
• 203b4d9 Update to use the common build system (#2883)
• a4f585b Remove UTF-8 byte order marks
• 8b2c7b2 Add v3 build submodule
• 2413c57 Throw when user tries to run a non-async test with a timeout
• 9ebc10c #2573: Opting out of AsyncTestSyncContext (v2)
• b060404 xunit/xunit#2880: Update XML documentation for string-based Assert.Equal (v2)
• 6bbf922 Add conditions to EquivalenceAssertsTests for XUNIT_IMMUTABLE_COLLECTIONS
• cab6a3e #2871: Inner exception stack trace is missing from Assert.Collection failure ...
• Additional commits viewable in compare view