Fix Z-index Issue on inline Findings Dropdown

What does this MR do and why?

Fix Z-index Issue on inline Findings Dropdown

This commit solves: #429033 (closed)

It fixes an Issue where the Findings Dropdown did not correctly overlap the file underneath on the Merge-Request Diff Page.

Changelog: fixed

Screenshots or screen recordings

Before	After
2023-11-28_11-23-50	2023-11-28_11-22-34

How to set up and validate locally

1. Enable sast_reports_in_inline_diff
2. Make sure your running an ultimate license or your GDK (how to)
3. Clone this repo: https://gitlab.com/jannik_lehmann/sast-inline-findings-example
4. Recreate this MR (exactly this MR since it's a UI edgecase) for SAST & CodeQuality findings: jannik_lehmann/cq-and-sast-test!5 (diffs)
5. Let the pipeline finish and the go checkout the diff page.

MR acceptance checklist

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