Bump GitVersion.Tool from 5.6.4 to 5.6.6 (#522)

Author: dependabot[bot]

Directory.Build.targets

@@ -11,7 +11,7 @@
     <PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" Version="1.0.0" PrivateAssets="All" />
   </ItemGroup>
   <ItemGroup>
-    <PackageReference Update="GitVersion.Tool" Version="5.6.4" />
+    <PackageReference Update="GitVersion.Tool" Version="5.6.6" />
     <PackageReference Update="JetBrains.ReSharper.CommandLineTools" Version="2020.3.2" />
     <PackageReference Update="ReportGenerator" Version="4.8.4" />
     <PackageReference Update="Rocket.Surgery.Nuke" Version="0.14.3" />