(development docs) Reinforce that GraphQL and REST API are both supported, equally

What does this MR do and why?

GraphQL and REST API are both supported, equally. There is no primary API.

We need to support both GraphQL, and REST APIs equally, the coverage will be added as required.

While it is true that Frontend prefers to consume GraphQL, the REST API will continue to be maintained as well.

See also:

• https://docs.gitlab.com/ee/api/rest/#compatibility-guidelines
• https://docs.gitlab.com/ee/development/fe_guide/frontend_goals.html#haml-by-default, https://docs.gitlab.com/ee/development/fe_guide/#overview
• https://docs.gitlab.com/ee/api/graphql/
• https://docs.google.com/presentation/d/1jyVsk5JrCFo_4C7mxgLQ--N74JqGgVonx6zs4XMhbg8/edit#slide=id.g11b34bfea9d_0_95 (internal)

Summary of discussions

1. Part of exposing any API is ensuring that it has enough coverage to perform the task that the customer needs to do.
2. Marking an API "primary" means the other API is not given enough attention. It's misinterpreted by some to mean no new REST APIs will be added.
3. https://handbook.gitlab.com/handbook/company/working-groups/api-vision/ did a survey and concluded that both REST and GraphQL continue to be used by significant sets of customers.
4. The REST API is not deprecated. The GraphQL API is not deprecated.
5. Both APIs have their pros and cons.
6. We can work to improve automation to support both APIs
7. FE prefers GraphQL - that does not change.
8. There are questions about what API should I implement for my feature. The answer is neither, we implement the feature that the customer requested. Likely, we will follow-up with a detailed guide on how to introduce new API operations/endpoints.

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