Improve accessible name of version dropdown in docs navbar (#36495)

Improve accessible name of version dropdown in docs navbar (#36495)

- add `aria-hidden="true"` to the "Bootstrap" text that is hidden on large screens
- add separate visually-hidden "Bootstrap", which will be part of the accessible name regardless of whether the other string is visible or not (on large screen)
- extra visually-hidden text to give some context - that the dropdown is about switching versions
- remove the redundant id/aria-labelledby for the dropdown