Bump SixLabors.ImageSharp from 1.0.4 to 2.1.8 in /PdfSharpCore

Bumps [SixLabors.ImageSharp](https://github.com/SixLabors/ImageSharp) from 1.0.4 to 2.1.8.
- [Release notes](https://github.com/SixLabors/ImageSharp/releases)
- [Commits](SixLabors/ImageSharp@v1.0.4...v2.1.8)

---
updated-dependencies:
- dependency-name: SixLabors.ImageSharp
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

PdfSharpCore/PdfSharpCore.csproj

@@ -47,7 +47,7 @@ PdfSharpCore is a partial port of PdfSharp.Xamarin for .NET Core Additionally Mi
   </ItemGroup>
   <ItemGroup>
     <PackageReference Include="SharpZipLib" Version="1.3.3" />
-    <PackageReference Include="SixLabors.ImageSharp" Version="1.0.4" />
+    <PackageReference Include="SixLabors.ImageSharp" Version="2.1.8" />
     <PackageReference Include="SixLabors.Fonts" Version="1.0.0-beta17" />
   </ItemGroup>