Bump SixLabors.ImageSharp in /src/Tools/PaletteBuilder (#83)

Bumps [SixLabors.ImageSharp](https://github.com/SixLabors/ImageSharp) from 1.0.2 to 3.1.3.
- [Release notes](https://github.com/SixLabors/ImageSharp/releases)
- [Commits](SixLabors/ImageSharp@v1.0.2...v3.1.3)

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

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

src/Tools/PaletteBuilder/PaletteBuilder.csproj

@@ -9,7 +9,7 @@
 
   <ItemGroup>
     <PackageReference Include="Microsoft.ML" Version="1.5.4" />
-    <PackageReference Include="SixLabors.ImageSharp" Version="1.0.2" />
+    <PackageReference Include="SixLabors.ImageSharp" Version="3.1.3" />
   </ItemGroup>
 
   <ItemGroup>