Updates variable value in Entry::Variable to be string rather than array and adds options

What does this MR do and why?

Improves based on feedback: #363660 (comment 1189635268). Additionally significantly simplifies variables implementation.

FROM:

variables:
  DEPLOY_ENVIRONMENT:
    value: 
      - "staging"  #  staging should appear by default
      - "canary" # Canary is the second option in a dropdown list
      - "production" # Production is the third option in a dropdown list
    description: "The deployment target. Change this variable to 'canary' or 'production' if needed."

TO:

variables:
  DEPLOY_ENVIRONMENT:
    value: staging
    options: 
      - "staging"
      - "canary"
      - "production"
    description: "The deployment target. Change this variable to 'canary' or 'production' if needed."

Validation that value is contained in options:

Run pipeline page:

Job log (I selected production):

MR acceptance checklist

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