Move pages_path_prefix to pages.path_prefix

What does this MR do and why?

Move pages_path_prefix to pages.path_prefix

After some conversation about how to enable pages multiple jobs, we decided to, for now, only rename the pages_path_prefix to pages.path_prefix.

pages_path_prefix was introduced as a root job option, but now that we the vision that we want to concentrate all pages build options under the same pages: option for better organization and discoverability.

This will not have any impacts as the pages_path_prefix options is currently behind a disabled feature flag and undocumented. The documentation will be added on another MR.

Related to:

• Enable multiple pages jobs (#427755 - closed)
• Improve `pages:` job option to be less verbose (#428018)

FAQ

1. Why are we adding a pages option that can only be used on jobs named pages?

   Yes, this is a bit weird at first, but this is a step towards Enable multiple pages jobs (#427755 - closed). In intend to replace the job name requirement by this new pages option.

2. Shouldn't we be updating documentation as well?

   The original pages_path_prefix documentation MR ( !130314 (merged)) wasn't finished when we decided to rename the field. So, I'll update that MR once this is merged with all the new information.

Screenshots or screen recordings

Screen_Recording_2023-10-17_at_18.51.08

How to set up and validate locally

1. You need a license (https://about.gitlab.com/handbook/developer-onboarding/#working-on-gitlab-ee-developer-licenses)

2. You need GitLab Runner (https://gitlab.com/gitlab-org/gitlab-development-kit/-/blob/main/doc/howto/runner.md?ref_type=heads)

3. Enable pages_multiple_versions_setting feature flag

4. Create a public project (to avoid having to configure private access) using the html template (root/html for example`)

5. Run the pipeline on that project (to ensure the pages is created)

6. Navigate to Settings > Pages (http://gdk.test:3000/root/html/pages, for example)

   • Disable Force HTTPS (requires valid certificates)
   • Enable Use multiple versions
   • [optional] Click on the Access pages URL to see your GitLab Pages site 👀

7. Edit your gitlab-ci.yml adding the pages_path_prefix to the pages job:

   pages:
     stage: deploy
     pages:
       path_prefix: '$CI_COMMIT_BRANCH' # mr<iid>, like `mr123`
     script:
     - echo 'Nothing to do...'
     artifacts:
       paths:
       - public
     only:
     - master

8. Run the pipeline again

9. Go to rails console (bundle exec rails console) and check if the last PagesDeployment has the given prefix:

   [dev](main)> PagesDeployment.last.path_prefix
   => "master"

10. Success! 🎉

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.