Use fluid layout for merge request list

Problem to Solve

Apply fluid layout to merge request lists (both project and group level) to increase available space

Related Work

This follows the approach implemented in !209478 (merged) which made issue lists use fluid width. The same pattern should be applied to merge request lists to:

• Increase available horizontal space
• Move labels inline when they fit given the additional space
• Align with the overall design direction for list consolidation
• Ensure consistency across different list types (issues, merge requests, etc.)

Points of concern

• I would love to see more structure and use of additional space on the widest view. For example grouping up the biggest parts like labels into columns so they're easier to scan through. Right now it only creates more space between left and right columns, which for me makes it harder to work with.
• Long lines for titles could be hard to read
• Need to balance temptation to add more while considering smaller viewports
• More space could allow for better structure and easier alignment of MR lists and add more info such as MR stats.