Bump Codeuctivity.ImageSharpCompare and SixLabors.ImageSharp in /src (#289)

Bumps [Codeuctivity.ImageSharpCompare](https://github.com/Codeuctivity/ImageSharp.Compare) and [SixLabors.ImageSharp](https://github.com/SixLabors/ImageSharp). These dependencies needed to be updated together.

Updates `Codeuctivity.ImageSharpCompare` from 4.0.306 to 4.0.311
- [Release notes](https://github.com/Codeuctivity/ImageSharp.Compare/releases)
- [Commits](Codeuctivity/ImageSharp.Compare@4.0.306...4.0.311)

Updates `SixLabors.ImageSharp` from 3.1.8 to 3.1.8
- [Release notes](https://github.com/SixLabors/ImageSharp/releases)
- [Commits](SixLabors/ImageSharp@v3.1.8...v3.1.8)

---
updated-dependencies:
- dependency-name: Codeuctivity.ImageSharpCompare
  dependency-version: 4.0.311
  dependency-type: direct:production
  update-type: version-update:semver-patch
- dependency-name: SixLabors.ImageSharp
  dependency-version: 3.1.8
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

Author: dependabot[bot]

src/Directory.Packages.props

@@ -4,7 +4,7 @@
     <CentralPackageTransitivePinningEnabled>true</CentralPackageTransitivePinningEnabled>
   </PropertyGroup>
   <ItemGroup>
-    <PackageVersion Include="Codeuctivity.ImageSharpCompare" Version="4.0.306" />
+    <PackageVersion Include="Codeuctivity.ImageSharpCompare" Version="4.0.311" />
     <PackageVersion Include="MarkdownSnippets.MsBuild" Version="27.0.2" />
     <PackageVersion Include="Microsoft.NET.Test.Sdk" Version="17.13.0" />
     <PackageVersion Include="NUnit" Version="4.3.2" />