chore: adjust ImageSharp version and remove SourceLink refs

Downgrade SixLabors.ImageSharp to 3.1.12 in central package management and
remove redundant SourceLink package references from IconHelper.csproj.

Author: matt-edmondson

Directory.Packages.props

@@ -7,7 +7,7 @@
     <PackageVersion Include="Microsoft.SourceLink.AzureRepos.Git" Version="10.0.300" />
     <PackageVersion Include="Microsoft.SourceLink.GitHub" Version="10.0.300" />
     <PackageVersion Include="Polyfill" Version="9.3.0" />
-    <PackageVersion Include="SixLabors.ImageSharp" Version="4.0.0" />
+    <PackageVersion Include="SixLabors.ImageSharp" Version="3.1.12" />
     <PackageVersion Include="ktsu.Extensions" Version="1.5.9" />
     <PackageVersion Include="ktsu.CaseConverter" Version="1.3.6" />
   </ItemGroup>

IconHelper/IconHelper.csproj

@@ -12,8 +12,6 @@
     <PackageReference Include="CommandLineParser" />
     <PackageReference Include="ktsu.CaseConverter" />
     <PackageReference Include="ktsu.Extensions" />
-  <PackageReference Include="Microsoft.SourceLink.AzureRepos.Git" />
-  <PackageReference Include="Microsoft.SourceLink.GitHub" />
   <PackageReference Include="Polyfill" VersionOverride="9.7.1" />
     <PackageReference Include="SixLabors.ImageSharp" />
   </ItemGroup>