Fix table inconsistent border if caption exists

[dgrammatiko]

Changes

The selector :not(:first-child) is changed to tbody:first-of-type so that the border-top has consistent results if there is or not a caption element

The problem is reproducible if the table has a caption element but the caption is visually hidden

For reference: joomla/joomla-cms#35766 (comment)

[ffoodd]

That's not appropriate since we want this border for each tbody, precisely. FYI there's an open issue related to this specific CSS when using colgroup element, too: #35152.

I try another way around.

[dgrammatiko]

That's not appropriate since we want this border for each tbody, precisely.

Ok, I think I can reduce the changes hereto affect only the caption element