Add API::AccessRequests to OpenAPI V2

What does this MR do and why?

This adds the API::AccessRequests endpoints to the OpenAPI V2 documentation.

Pasting the doc file to https://editor.swagger.io/ shows two errors. But it is okay to ignore them for now, as described in https://gitlab.com/gitlab-org/gitlab/-/issues/377941:

Semantic error at paths./api/v4/groups/{id}/access_requests/{user_id}/approve.put
Operations with parameters of "in: formData" must include "application/x-www-form-urlencoded" or "multipart/form-data" in their "consumes" property
Jump to line 50

Semantic error at paths./api/v4/projects/{id}/access_requests/{user_id}/approve.put
Operations with parameters of "in: formData" must include "application/x-www-form-urlencoded" or "multipart/form-data" in their "consumes" property
Jump to line 184

The doc/api/openapi/openapi_v2.yaml is autogenerated using the rake gitlab:openapi:generate task.

Related issue: https://gitlab.com/gitlab-org/gitlab/-/issues/376855

How to set up and validate locally

1. Visit https://gitlab.com/gitlab-org/gitlab/-/blob/c2e2e3ab7e9e4affd613d3ec5cb6db9f755735e7/doc/api/openapi/openapi_v2.yaml#/access_requests
2. Explore the access_requests section

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.