fix: src/jcdcdev.Umbraco.ExtendedMarkdownEditor.TestSite/jcdcdev.Umbraco.ExtendedMarkdownEditor.TestSite.csproj to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-DOTNET-UMBRACOCMSCORE-10300811

Author: snyk-bot

src/jcdcdev.Umbraco.ExtendedMarkdownEditor.TestSite/jcdcdev.Umbraco.ExtendedMarkdownEditor.TestSite.csproj

@@ -6,7 +6,7 @@
     </PropertyGroup>
 
     <ItemGroup>
-        <PackageReference Include="Umbraco.Cms" Version="15.3.1" />
+        <PackageReference Include="Umbraco.Cms" Version="15.4.2" />
         <PackageReference Include="uSync" Version="15.1.6"/>
     </ItemGroup>