Update benchmarks to use Colourful 2.0.0 (allocation free)

tests/ImageSharp.Benchmarks/ImageSharp.Benchmarks.csproj

@@ -17,7 +17,7 @@
   </ItemGroup>
   <ItemGroup>
     <PackageReference Include="BenchmarkDotNet" Version="0.10.14" />
-    <PackageReference Include="Colourful" Version="1.2.2" />
+    <PackageReference Include="Colourful" Version="2.0.0" />
     <PackageReference Include="SixLabors.Shapes.Text" Version="1.0.0-dev000081" />
     <PackageReference Include="System.Drawing.Common" Version="4.5.0" />
   </ItemGroup>