Fix when include has rules with invalid rule

What does this MR do and why?

Bug description: #341488 (closed)

The include:rules keyword only supports if conditions. When you use other conditions, like exists, the GitLab CI config file show page returns 500. Also, the created pipeline raises an undefined error.

This MR fixes that error and raises a proper message.

Screenshots or screen recordings

Before

After

How to set up and validate locally

include:
  - local: configs/builds.yml
    rules:
      - exists:
        - '$INCLUDE_BUILDS'

test:
  stage: test
  script: exit 0

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