Tables headers should have no background out of the box

Description

When I create a table using only <gl-table> I get a gray background in the header which is not Pajamas compliant (see https://design.gitlab.com/components/table). This would force me to add some custom classes, but it would be nicer to get the right appearance out of the box:

In some cases where we force our styles with gl-bg-transparent!, looks like this doesn’t address the original cause of the styles.