Resolve "Merge request refactor: Title and copy to clipboard button are behind the action buttons"

What does this MR do?

Solves the problem of overlapping file name and file actions in the Changes tab of the MR page.

Are there points in the code the reviewer needs to double check?

To avoid from affecting other unwanted places, I tried to keep it contextualised to .file-holder.

I also changed the breakpoint in which the File actions started to be hidden:

• BS4 updated its breakpoints and small goes up to 575px now
• Before we were hiding the FIle actions below that line (previous medium breakpoint)
• It's now hidden only on viewports narrower than 575px wide (as before BS4 update)

Why was this MR needed?

Long files names or narrow screens would cause the two elements to overlap, making the file name unreadable.

Screenshots (if relevant)

Before
Now
Now (small screens)	This already happened, but it was happening in wider viewports, which was a side-effect of BS4 update.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Conform by the code review guidelines
  • Has been reviewed by a Frontend maintainer
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them

What are the relevant issue numbers?

Closes #48603 (closed)