Variable expansion in deployment_tier

What does this MR do and why?

Related #365402.

Support variable expansion in the environment:deployment_tier field of a job in the .gitlab-ci.yml.

⚠️ In order to preserve validation integrity, this blocked until GitLab has upgraded to Rails 7.1, see #365402 (comment 2200490852) for details. (Ref: Blocked by the apic Rails 7.1 upgrade (&11470))

MR acceptance checklist

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Before	After

How to set up and validate locally

1. Run GDK and prepare project.
2. Define a pipeline. For example,

deploy:
  variables:
    ENV_TIER: production
  environment:
    name: env3
    deployment_tier: $ENV_TIER
  script:
    - echo 'deploy'
    - echo $CI_ENVIRONMENT_TIER
    - echo $ENV_TIER

3. Run the pipeline. And confirm that the deployment tier is correctly expanded, and the environment tier is updated.