Dotnet Templates: Update default UmbracoVersion template value using MSBuild target (closes #21889)

[AndyButland]

Description

The umbraco-extension template defaults its Umbraco package references to *, which causes CI builds to pull the latest version instead of the version matching the installed template package.

The can cause the problem outlined in #21889.

To fix I've applied the same JsonPathUpdateValue MSBuild fix that was added for the umbraco project template in #13481 — updating $.symbols.UmbracoVersion.defaultValue from * to $(PackageVersion) during dotnet pack

Testing

Firstly run the following to install a new extension project:

dotnet new install Umbraco.Templates::17.2.0
dotnet new umbraco-extension --name TestExtension
cd .\TestExtension\
code .

Open up TestExtension.csproj and note the project references all use "*":

<ItemGroup>
  <PackageReference Include="Umbraco.Cms.Web.Website" Version="*" />
  <PackageReference Include="Umbraco.Cms.Web.Common" Version="*" />
  <PackageReference Include="Umbraco.Cms.Api.Common" Version="*" />
  <PackageReference Include="Umbraco.Cms.Api.Management" Version="*" />
</ItemGroup>

Now run a dotnet pack from this PR. Perhaps the easiest way is save the *.nupkg files generated from this PR into a local folder.

Delete the TestExtension folder and run:

cd ../
dotnet new install Umbraco.Templates::17.3.0--rc.preview.109.g8c069b7 --nuget-source c:\repos\nuget\Umbraco.Templates.17.3.0--rc.preview.109.g8c069b7.nupkg
dotnet new umbraco-extension --name TestExtension
cd .\TestExtension\
code .

Open up TestExtension.csproj and note the project references all use the matching version:

<ItemGroup>
  <PackageReference Include="Umbraco.Cms.Web.Website" Version="17.3.0--rc.preview.109.g8c069b7" />
  <PackageReference Include="Umbraco.Cms.Web.Common" Version="17.3.0--rc.preview.109.g8c069b7" />
  <PackageReference Include="Umbraco.Cms.Api.Common" Version="17.3.0--rc.preview.109.g8c069b7" />
  <PackageReference Include="Umbraco.Cms.Api.Management" Version="17.3.0--rc.preview.109.g8c069b7" />
</ItemGroup>

[Copilot]

Pull request overview

This pull request updates the MSBuild target in Umbraco.Templates.csproj to dynamically set the UmbracoVersion symbol's default value to the current package version during the build process. This ensures that the .NET templates use the correct Umbraco version by default when developers create new projects or extensions.

Changes:

• Adds a JsonPathUpdateValue task to update the UmbracoVersion symbol's defaultValue in template.json files during the build

[Zeegaan]

LGTM 🚀
Thank you for the very detailed testing steps 💪