Resolve "Reveal Variables button may overrun width of screen"

What does this MR do?

Fixes a regression that causes columns in the Project Variables table to overrun the bounds of the table and be hidden. %-based widths are tricky and limiting with html tables. So I set a fixed-width for the variable-key column and set a media query to adjust the column for smaller screens.

It seemed preferable that text in the key column wraps to the next line on small screens. I could imagine scenarios where variables are prefixed with similar values, making them difficult to differentiate on small screens. Reviewers, WDYT?

cc: @annabeldunstone @stanhu

Does this MR meet the acceptance criteria?

• Changelog entry added
• All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #25780 (closed)