Bump VSThreadingVersion from 17.11.20 to 17.12.19 (#799)

Bumps `VSThreadingVersion` from 17.11.20 to 17.12.19.

Updates `Microsoft.VisualStudio.Threading` from 17.11.20 to 17.12.19
- [Release notes](https://github.com/microsoft/vs-threading/releases)
- [Commits](https://github.com/microsoft/vs-threading/commits)

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Directory.Packages.props

@@ -6,7 +6,7 @@
     <CentralPackageTransitivePinningEnabled>true</CentralPackageTransitivePinningEnabled>
     <BenchmarkDotNetVersion>0.13.12</BenchmarkDotNetVersion>
     <AspNetCoreVersion>2.2.0</AspNetCoreVersion>
-    <VSThreadingVersion>17.11.20</VSThreadingVersion>
+    <VSThreadingVersion>17.12.19</VSThreadingVersion>
   </PropertyGroup>
   <ItemGroup>
     <PackageVersion Include="BenchmarkDotNet.Diagnostics.Windows" Version="$(BenchmarkDotNetVersion)" />