fix: src/TestSite.13/TestSite.13.csproj to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-DOTNET-SIXLABORSIMAGESHARP-11338623

Author: snyk-bot

src/TestSite.13/TestSite.13.csproj

@@ -7,7 +7,7 @@
     </PropertyGroup>
 
     <ItemGroup>
-        <PackageReference Include="Umbraco.Cms" Version="13.10.1"/>
+        <PackageReference Include="Umbraco.Cms" Version="13.12.0"/>
         <PackageReference Include="uSync" Version="13.2.7"/>
     </ItemGroup>