Replace "header-content.menu-expanded" styles with ".top-nav-responsive-open"

Description

Once the :combined_menu feature flag is removed #324086 (closed), we can consolidate the styling for .menu-expanded and .top-nav-responsive-open:

• https://gitlab.com/gitlab-org/gitlab/blob/58cad116882ad5abe202a0ba8e79b023c8c3f1d4/app/assets/stylesheets/framework/header.scss#L109
• https://gitlab.com/gitlab-org/gitlab/blob/58cad116882ad5abe202a0ba8e79b023c8c3f1d4/app/assets/stylesheets/framework/header.scss#L674

This will also resolve this comment in main.js.

Implementation notes

It'll probably be a lot easier to replace the .hide-when-menu-expanded with .hide-when-top-nav-responsive-open. Also, please note that the .top-nav-responsive-open is applied to the body. We can then also remove the JavaScript that is toggling the .menu-expanded.