GlModal: Fix accessibility violations by avoiding use of header and footer tags

Summary

In Project Studio UI, the page header as defined in super_sidebar/components/super_topbar.vue uses <header>, which means that in presence of modal in the DOM, page will have two <header>, leading to a11y violation (eg; https://gitlab.com/gitlab-org/gitlab/-/issues/578455).

A fix for this is to use <div> for both header and footer contents with appropriate aria-label values, the BModal supports header-tag and footer-tag props already that we can utilize here.