Raise error if gitlab-ci.yml contains both old and new Requirements versions

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backend bit of #388524. See #382141 (comment 1242242288) for context.

Currently the following is a valid .gitlab-ci.yml:

requirements_confirmation:
  when: manual
  allow_failure: false
  script:
    - mkdir tmp
    - echo "{\"20\":\"passed\"}" > tmp/requirements_v2.json
    - echo "{\"2\":\"passed\"}" > tmp/requirements.json
  artifacts:
    reports:
      requirements: tmp/requirements.json
      requirements_v2: tmp/requirements_v2.json

However, this only runs the v2 part of the job and the requirements bit is ignored.

There should be an error:

• in a CI linter
• perhaps when running the job itself

We'll have to liaise with devopsverify to ensure it is consistent with existing paradigms.