Bump SixLabors.ImageSharp from 1.0.4 to 3.1.3 in /VDF.Core

Bumps [SixLabors.ImageSharp](https://github.com/SixLabors/ImageSharp) from 1.0.4 to 3.1.3.
- [Release notes](https://github.com/SixLabors/ImageSharp/releases)
- [Commits](SixLabors/ImageSharp@v1.0.4...v3.1.3)

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

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

VDF.Core/VDF.Core.csproj

@@ -12,7 +12,7 @@
     <PackageReference Include="FFmpeg.AutoGen" Version="6.0.0.2" />
     <PackageReference Include="Mono.Posix.NETStandard" Version="5.20.1-preview" />
     <PackageReference Include="protobuf-net" Version="3.2.26" />
-    <PackageReference Include="SixLabors.ImageSharp" Version="1.0.4" />
+    <PackageReference Include="SixLabors.ImageSharp" Version="3.1.3" />
   </ItemGroup>
 
 </Project>