Reset webpack path for Mermaid iFrames

What does this MR do and why?

This is backport MR for !123006 (merged) which fixed #413596 (closed)!

This MR sets correct webpack path for Mermaid iFrames to render it correctly. This was broken after this upgrade: !115885 (merged) possibly because it tries to load more chunks.

The sandbox iFrame 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.
• The original MR has been deployed to GitLab.com (not applicable for documentation or spec changes).
• This MR has a severity label assigned (if applicable).
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:package-and-test-ee job has either succeeded or been approved by a Software Engineer in Test.

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