Fix inconsistent production environment definition on VSA

What does this MR do?

We recently introduced the first class concept for production environments. As you can read in the docs, this feature allows users to explicitly define production environment regardless of the environment name. All features in GitLab that depends on production environments should identify the production environments via this SSOT feature, instead of building a unique business logic per domain.

This MR improves the Allow other names for VSA production stage by replacing the obsolete PRODUCTION_ENVIRONMENT_IDENTIFIERS by the new feature. This subsequently fixes a bug that VSA doesn't work for production environments which don't follow the current matching pattern, such as gprd.

FYI, you can see the new feature's regexp in https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/models/environment.rb#L469.

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