Use VERSION for OpenAPI 3.0 specification

What does this MR do and why?

Use Gitlab::VERSION for OpenAPI 3.0 specification version.

References

• https://gitlab.com/gitlab-com/gl-infra/api-docs/-/work_items/6+
• https://swagger.io/specification/#info-version

How to set up and validate locally

1. In rails console check the OpenAPI 3.0 specification version

   Gitlab::GrapeOpenapi.configuration.info.version
   # => "19.1.0-pre"

2. Confirm version is same as VERSION

   Gitlab::GrapeOpenapi.configuration.info.version == Gitlab::VERSION
   # => true

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.