Update Pages CI/CD template examples to default branch

What does this MR do?

We have a lot of template examples for various deployments with Pages. Many have associated projects at: https://gitlab.com/pages (I'll update these separately, when I get access to the top-level group. Edit: I now have access, so after this merges I'll update those templates to match)

As part of the linked issue, we should make sure they all point to $CI_DEFAULT_BRANCH and not a hardcoded default branch like master.

Changes:

• Before:

  only:
    - master

  After:

  rules:
    - if: $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH

• For templates that used except, we shouldn't jump straight to rules, see: !56301 (comment 526788078). So:

  Before:

  only:
    - master
  
  except:
    - master

  After:

  only:
    variables:
      - $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH
  
  except:
    variables:
      - $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH

Related to #324131 (closed)

Does this MR meet the acceptance criteria?

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