Add pipeline syntax check when publish and pages.publish both specified

What does this MR do and why?

Add pipeline syntax check when publish and pages.publish both specified

Changelog: fixed

References

Add pipeline syntax check when pages.publish an... (#519514 - closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Follow instructions mentioned here and in step 3 select Pages/Plain HTML template.

2. On the left sidebar, select Build > Pipeline editor.

3. Replace context with below

   image: busybox
   
   pages:
     stage: deploy
     script:
       - echo "The site will be deployed to $CI_PAGES_URL"
     publish: public
     pages:
       publish: 'public'

4. Verify an error pages job: use either pages:publish or pages:pages:publish is shown on top bar.

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.