Resolve "Z-Index Issue on Pipeline Dropdown"

What does this MR do?

Lowers the z-index of the .merge-request-tabs-holder class from 300 to 250 to avoid collision with the mini pipeline status dropdown.

For context, this z-index value was bumped up from 200 to 300 in this commit: https://gitlab.com/gitlab-org/gitlab-ce/commit/d17de657fe782eb838545aabeee00285e181dce1#f06c7f313a79a2b350d89b9c18b36ab8986980a5_812_812.

What are the relevant issue numbers?

gitlab-org/gitlab-ce#58999

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer

Closes #58999 (closed)