some more fixes for the simpler pa11y errors referenced in recent https://github.com/twbs/bootstrap/pull/29315 discussion
note that some other outstanding errors (and even some of the tweaks here) are not necessarily hard failures (e.g. the scope attribute on <th> is not really mandatory/necessary, and the need for a submit button when the only form control present is a single text/search input is debatable) ... seems pa11y is a bit overzealous with its errors when they're not necessarily failures