fix(openapi): declare an absolute server for the management API

What does this MR do and why?

api/openapi/v1.yaml declared only a relative server (url: /), which resolves against whatever origin serves the document. That is correct for this project's own Redoc build on its Pages site, and wrong anywhere else: in a renderer hosted elsewhere, "Try it" sends requests to the renderer's own host instead of to the registry.

This matters now because the management API is about to be rendered in GitLab's interactive API reference at https://api.gitlab.com/rest/ (MR that adds it: https://gitlab.com/gitlab-com/gl-infra/api-docs/-/merge_requests/203, won't be merged until AR's beta), where a relative server would point every request at api.gitlab.com.

Adds an absolute templated entry ahead of the relative one, defaulting to the dedicated domain ADR-009 names. Templated rather than hardcoded so self-managed operators can substitute their own host.

The relative entry stays, for two reasons. Same-origin consumers keep working unchanged. And the contract tests route requests through kin-openapi's gorillamux router, which matches on the declared servers: with an absolute-only list it stops matching their http://contract.test requests and every contract assertion fails with "no matching operation was found".

Please correct the default host if artifact-registry.gitlab.com is not what production will serve.

References

Related to: gitlab-org/gitlab#608507 (closed)