fix: src/templates/UmbracoPackageTemplate/src/UmbracoPackageTemplate.TestSite/UmbracoPackageTemplate.TestSite.csproj to reduce vulnerabilities

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

Author: snyk-bot

src/templates/UmbracoPackageTemplate/src/UmbracoPackageTemplate.TestSite/UmbracoPackageTemplate.TestSite.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>