Bump SixLabors.ImageSharp from 1.0.3 to 2.1.8 in /tools/objexport

Bumps [SixLabors.ImageSharp](https://github.com/SixLabors/ImageSharp) from 1.0.3 to 2.1.8.
- [Release notes](https://github.com/SixLabors/ImageSharp/releases)
- [Commits](SixLabors/ImageSharp@v1.0.3...v2.1.8)

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

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

tools/objexport/objexport.fsproj

@@ -9,7 +9,7 @@
   <ItemGroup>
     <PackageReference Include="Newtonsoft.Json" Version="13.0.1" />
     <PackageReference Include="RCT2Tools.ObjectData" Version="1.2.3" />
-    <PackageReference Include="SixLabors.ImageSharp" Version="1.0.3" />
+    <PackageReference Include="SixLabors.ImageSharp" Version="2.1.8" />
   </ItemGroup>
   <ItemGroup>
     <Compile Include="AssemblyInfo.fs" />