Fix openapi viewer for relative url instances

This is %15.10 backport MR for !114542 (merged).

What does this MR do and why?

Fixes #381047 (comment 1296098032).

This fixes the openapi viewer for instances configured with root-relative url.

This has been broken because the iframe sandbox loads on a separate origin which doesn't have access to gon.relative_url_root to load the webpack chunk properly.

MR acceptance checklist

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• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
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• This MR has a severity label assigned (if applicable).
• Ensure the e2e:package-and-test job has either succeeded or been approved by a Software Engineer in Test.

Note to the merge request author and maintainer

The process of backporting bug fixes into stable branches is tracked as part of an internal pilot. If you have questions about this process, please:

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