Warning NU1608 with current version if FsUnit.xUnit

[abelbraaksma]

As in the title, we see:

Warning NU1608: Detected package version outside of dependency constraint: FsUnit.xUnit 5.5.0 requires xunit (>= 2.5.3 && < 2.6.0) but version xunit 2.6.1 was resolved.

Reported here: fsprojects/FsUnit#253