Permit users to configure pages `expire_in` in gitlab-ci.yml

What does this MR do and why?

This MR adds the expire_in property to the pages job in .gitlab-ci.yml to override the instance default for extra deployments.

Screenshots or screen recordings

(No visual changes)

How to set up and validate locally

1. Set up your GDK to support Runners and Pages
2. Enable the multi version feature flag with rails c and then Feature.enable("pages_multiple_versions_setting")
3. Enable the new pages UI feature flag with rails c and then Feature.enable("new_pages_ui")
4. Create a Pages project by creating a new project with the "pages/Plain HTML" template.
5. Replace the below config in the .gitlab-ci.yml:

image: busybox

pages:
  stage: deploy
  script:
    - echo "Pages accessible through ${CI_PAGES_URL}/${PAGES_PREFIX}"
  variables:
    PAGES_PREFIX: "" # no prefix by default (master)
  pages:
    path_prefix: "$PAGES_PREFIX"
    expire_in: 60 # one minute
  artifacts:
    paths:
    - public
  rules:
    - if: $CI_COMMIT_BRANCH == "master" # ensure to run on master (with default PAGES_PREFIX)
      variables:
        PAGES_PREFIX: '' # prefix with _stg for the staging branch
    - if: $CI_PIPELINE_SOURCE == "merge_request_event" # conditionally change the prefix on Merge Requests
      variables:
        PAGES_PREFIX: 'mr$CI_MERGE_REQUEST_IID' # prefix with the mr<iid>, like `mr123`

5. Ensure the pipeline runs successfully and you have a pages deployment
6. Enable multiple deployments for your new project
7. Create at least one MR in the project. Ensure each MR's pipeline creates a Pages deployment.
8. In the project's sidebar go to Deploy > Pages, view the MR's deployments, click on the chevron and confirm that they now show an expiry date and time that aligns with the expire_in property from the .gitlab-ci.yml
9. Wait until that expiry date has passed* and confirm that the deployments have been removed (note that they will go first into a "stopped" state before they are permanently removed)

• the cron job only runs at every 10 minute mark, so you may have to wait for the next job run, or update the cron schedule of the deactivate_expired_deployments_cron_worker job in config/initializers/1_settings.rb

Related to #456478 (closed)