Update OpenAPI CI job to also use the v3 rake task

What does this MR do and why?

1. Updates the CI job to have it also verify that the generated OAPI3 spec is up-to-date in the repository, apart from doing it only for the OAPI2 like before. This ensures that any changes to the API are reflected in both the OAPI 2 and 3 specifications before merging.
2. Replaces the dynamic Gitlab.config.gitlab.url URL with the static 'https://gitlab.com/api' in the in OAPI3 initializer to ensure consistent OAPI specification output across environments.

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

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.

Related to #581173 (closed)