Update right sidebar top position for MR sidebar on small screens

What does this MR do and why?

Update right sidebar top position for MR sidebar on small screens

• Update top position for .right-sidebar-merge-requests to include --top-bar-height
• Maintain existing top position for .right-sidebar:not(.right-sidebar-merge-requests)
• Remove padding-right moving MR review bar action buttons when .right-sidebar-expanded

Screenshots or screen recordings

Before	After

Mobile	Desktop

How to set up and validate locally

• View MR e.g http://127.0.0.1:3000/flightjs/Flight/-/merge_requests/4
• Scale viewport down to small width
• Expand/Collapse right sidebar
• Review top bar (with breadcrumbs) displays above the right sidebar contents (you can still collapse sidebar)

Note: this assumes you have the moved MR sidebar with Feature.enable(:moved_mr_sidebar), otherwise there will be no change see screenshots below:

Mobile	Desktop

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.