Bump System.Threading.Tasks.Dataflow from 4.9.0 to 6.0.0 (#749)

Author: dependabot[bot]

src/NerdBank.GitVersioning.Tests/NerdBank.GitVersioning.Tests.csproj

@@ -36,7 +36,7 @@
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     <PackageReference Include="Microsoft.Build.Utilities.Core" Version="15.1.548" ExcludeAssets="runtime" />
     <PackageReference Include="Microsoft.Build.Locator" Version="1.4.1" />
-    <PackageReference Include="System.Threading.Tasks.Dataflow" Version="4.9.0" />
+    <PackageReference Include="System.Threading.Tasks.Dataflow" Version="6.0.0" />
     <PackageReference Include="Xunit.Combinatorial" Version="1.2.7" />
     <PackageReference Include="xunit" Version="2.4.1" />
     <PackageReference Include="coverlet.collector" Version="3.1.2" />