Add environment key to pages jobs

What does this MR do and why?

Describe in detail what your merge request does and why.

This adds the environment keyword to the pages job in the following CI/CD template files:

• https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/ci/templates/Pages/Brunch.gitlab-ci.yml
• https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/ci/templates/Pages/Jigsaw.gitlab-ci.yml
• https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/ci/templates/Pages/SwaggerUI.gitlab-ci.yml

pages:
  ...
  environment: production

The related issues are #370967 (closed) and &8586

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