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GH-43058: [C#] Revert upgrade of Xunit from 2.8.0 to 2.8.1 (#43074)

### Rationale for this change

See #43058, this release broke compatibility with xunit.skippablefact and caused skipped tests to be treated as failures.

### What changes are included in this PR?

This reverts commit ef3d4670e70a40f46e6acdd7041b5ee7edb16a42.

### Are these changes tested?

Yes

### Are there any user-facing changes?

No
* GitHub Issue: #43058

Authored-by: Adam Reeve <adreeve@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>