Bump GitVersion.Tool from 5.6.6 to 5.6.8

Bumps [GitVersion.Tool](https://github.com/GitTools/GitVersion) from 5.6.6 to 5.6.8.
- [Release notes](https://github.com/GitTools/GitVersion/releases)
- [Changelog](https://github.com/GitTools/GitVersion/blob/main/BREAKING_CHANGES.md)
- [Commits](GitTools/GitVersion@5.6.6...5.6.8)

Signed-off-by: dependabot[bot] <[email protected]>

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="5.6.6" />
+    <PackageReference Update="GitVersion.Tool" Version="5.6.8" />
     <PackageReference Update="JetBrains.ReSharper.CommandLineTools" Version="2020.3.3" />
     <PackageReference Update="ReportGenerator" Version="4.8.6" />
     <PackageReference Update="Rocket.Surgery.Nuke" Version="0.14.3" />