Rename project default domain redirect to primary domain

Change request:

We should use Primary domain instead of Default URL , “Primary” is a common term in GitHub Pages, Netlify, etc... That conveys this domain is the one canonical location for a site. “Default URL” might still sound ambiguous.

Here is a proposition for the help text:

If selected, requests to your Pages site from any other domain (including the GitLab-provided domain) are redirected here. Use this to ensure all visitors (and search engines) see a single, consistent domain.