v4: Improve accessible name of version dropdown in docs navbar

• add sr-only "Bootstrap", which will be part of the accessible name
• extra sr-only text to give some context - that the dropdown is about switching versions
• remove the redundant id/aria-labelledby for the dropdown
• change the control from <a> to <button>

v4 port (and further tweaks) of https://github.com/twbs/bootstrap/pull/36495