Docs: Remove v3 to v4 page

Description

As part of the API Docs improvements #383847 (closed), I think we should remove the v3 to v4 reference.

Currently, the page redirects to https://gitlab.com/gitlab-org/gitlab-foss/-/blob/11-0-stable/doc/api/v3_to_v4.md

API V3 was unsupported from GitLab 9.5, released on August 22, 2017. API v3 was removed in GitLab 11.0. Time to move on.