Remove stage default from CI schema

What does this MR do and why?

For #341600 (closed) and #335961 (closed)

The following changes are added to the CI schema for the pipeline editor:

• Updated description for start_in keyword, as described in https://docs.gitlab.com/ee/ci/jobs/job_control.html#run-a-job-after-a-delay
• Removed "test" as a default stage name. Also added a minLength: 1 rule for stage so it checks that the value is not empty.

Screenshots or screen recordings

stage

Before	After
Before	After

start_in

How to set up and validate locally

1. Enable the schema_linting feature flag (rails c then Feature.enable(:schema_linting))
2. Go to the Pipeline Editor (CI/CD > Editor)
3. If you type stage into the editor, it should no longer prompt a default "stage" value. It should show the error if the user doesn't provide a value.
4. Typing an incorrect start_in value will show the updated description on hover.

Sample .gitlab-ci.yml content:

stages:
  - build

job:
  stage: build
  when: delayed
  start_in: 1 week
  script:
    echo "Hello World!"

MR acceptance checklist

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