[5.x] Fix border radius for data tables

[duncanmcclean]

This pull request fixes a small UI issue on some data tables where the border radius wouldn't be there.

Fixes #10760.

[jasonvarga]

This feels too heavy handed and may end up cutting off things somewhere.

I think this was introduced by #10266 (which I edited 😊 )

I think the proper fix would be to adjust the css rules in tables.css a bit so we don't have to override them all over the place. The :first-child stuff isn't really correct - it should factor in whether there's a header row or not.