Improve Tabs accessibility
Problem
The CI/CD design review discussion at 21 nov also indicated that tabs might be too subtle in their styling.
Issues such as gitlab#33813 (closed), gitlab#36125 (closed), gitlab#30648, and UXR insights such as https://gitlab.com/gitlab-org/uxr_insights/issues/807 indicate the same.
Proposal
Adjust the styling of tabs to be more accessible, more clear, similar to buttons.