GitLab RSS Feeds Failing Validation

The RSS feeds produced by GitLab are increasingly failing when read by clients. This is largely due to clients now enforcing RSS validation when consuming, for example requiring the use of absolute URLs rather than relative URLs which GitLab uses.

You can see this demonstrated on gitlab.com GitLab MR RSS feed where the atom feed reference is relative

<link href="/gitlab-org/gitlab/-/merge_requests.atom?state=opened" rel="self" type="application/atom+xml"/>

This is highlighted through W3.org's RSS validation tool: https://validator.w3.org/feed/check.cgi?url=https%3A%2F%2Fgitlab.com%2Fgitlab-org%2Fgitlab%2F-%2Fmerge_requests.atom#l4

There looks to have been work done in this space previously: gitlab-foss#653 (closed) but should be revisited to address the above.