* 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

(Fixes #22414) Rename for consistency `$custom-checkbox-radius`, `$custom-checkbox-checked-icon`, `$custom-checkbox-indeterminate-indicator-color`, `$custom-checkbox-indeterminate-icon`, `$custom-radio-radius`, `$custom-radio-checked-icon`, `$custom-select-sm-font-size`, to `$custom-checkbox-border-radius`, `$custom-checkbox-icon-checked`, `$custom-checkbox-indicator-indeterminate-color`, `$custom-checkbox-icon-indeterminate`, `$custom-radio-border-radius`, `$custom-radio-icon-checked`, `$custom-select-font-size-sm` respectively