Update outdated schema spec/fixtures/api/schemas/pipeline.json

The following discussion from gitlab-ce!32323 should be addressed:

• @splattael started a discussion: (+2 comments)

  Out of curiosity, do you know why this schema looks quite different from other schema definitions? 🤔

  It's only one using "$schema": "http://json-schema.org/draft-04/schema#".

  We're also missing required attributes along some others. I wonder why?