Fix site creation flow to not have Jekyll site engine by default

[sknep]

Adding a site repo to Pages from GitHub (rather than our template chooser) automatically sets the new site's file engine as Jekyll. It should be static or node, because nobody really uses Jekyll anymore, and the ruby stuff takes forever to install.

https://github.com/cloud-gov/pages-core/tree/main/frontend/pages/sites/new/index.jsx#L101

showAddNewSiteFields might not be properly set to true?
https://github.com/cloud-gov/pages-core/tree/main/frontend/pages/sites/new/AddRepoSiteForm.jsx

[afishman-2025]

implemented as part of #4872