Prevent mixin media-breakpoint-only from breaking when supplying the
first or last entry in $grid-breakpoints.

close #22338 by putting h4 text within links, but keep headings as is because i can't justify writing h3.h4 

wrap .bd-navbar position styles in supports query, remove prefix, and update linter settings for file

* Add explicit aria-label attributes to <svg> based links

As some browser/AT combinations don't seem to fully support grabbing the
`<title>` from inside the `<svg>` as the link text/name to announce
(e.g. Firefox+NVDA)

* Add <title> to Bootstrap logo <svg>

* Add <title> to menu <svg>

* Add explicit focusable="false" to <svg> icons

Due to a bug/"feature" in IE, `<svg>` elements are usually separately
focusable, which then results in links with `<svg>` icons being
essentially focused twice (once for the link, and once for the `<svg>`
inside, which is then also non-interactive)

Part of an update to align the naming schemes across our components.

- Renames .popover-title to .popover-header
- Renames .popover-content to .popover-body

Refs: #22092