Bump Markdig.Signed from 0.17.1 to 0.18.0 (#10887)

Bumps [Markdig.Signed](https://github.com/lunet-io/markdig) from 0.17.1 to 0.18.0.
- [Release notes](https://github.com/lunet-io/markdig/releases)
- [Changelog](https://github.com/lunet-io/markdig/blob/master/changelog.md)
- [Commits](xoofx/markdig@0.17.1...0.18.0)

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

src/Microsoft.PowerShell.MarkdownRender/Microsoft.PowerShell.MarkdownRender.csproj

@@ -9,7 +9,7 @@
   <ItemGroup>
   <!-- Source: https://github.com/lunet-io/markdig/ -->
     <ProjectReference Include="../System.Management.Automation/System.Management.Automation.csproj" />
-    <PackageReference Include="Markdig.Signed" Version="0.17.1" />
+    <PackageReference Include="Markdig.Signed" Version="0.18.0" />
   </ItemGroup>
 
 </Project>