Bump Microsoft.Build.Utilities.Core from 17.11.4 to 17.12.6 in /source

Bumps [Microsoft.Build.Utilities.Core](https://github.com/dotnet/msbuild) from 17.11.4 to 17.12.6.
- [Release notes](https://github.com/dotnet/msbuild/releases)
- [Changelog](https://github.com/dotnet/msbuild/blob/main/documentation/Changelog.md)
- [Commits](dotnet/msbuild@v17.11.4...v17.12.6)

---
updated-dependencies:
- dependency-name: Microsoft.Build.Utilities.Core
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

source/DefaultDocumentation/DefaultDocumentation.csproj

@@ -20,7 +20,7 @@
   </ItemGroup>
 
   <ItemGroup>
-    <PackageReference Include="Microsoft.Build.Utilities.Core" Version="17.11.4" />
+    <PackageReference Include="Microsoft.Build.Utilities.Core" Version="17.12.6" />
   </ItemGroup>
 
   <ItemGroup>