Resolve "Un-vendor CI templates"

What does this MR do?

The other side of this work gitlab-ci-yml!204 (merged)

• mv vendor/gitlab-ci-yml lib/gitlab/ci/templates
• add a spec that validates all the templates in this repo (equivalent of the spec we had in https://gitlab.com/gitlab-org/gitlab-ci-yml)
• update all docs so they no longer mention the old templates repo

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• [-] Conforms to the database guides

Closes #50289 (closed)