Add missing role="button" in .navbar's .dropdown-toggle

[XhmikosR]

Fixes #26830

[patrickhlauke]

oops, just realised perhaps intent was not to merge due to closeness of new release...sorry if that was the case/if that creates issues now :(

[XhmikosR]

Nah, this is good I believe; it's a trivial change, don't worry.

[thgh]

Why does Bootstrap use <a> instead of <button> for a button functionality?

[patrickhlauke]

mainly, for ease/consistency of styling. as it's then still exposed correctly thanks to the role="button", it's not a major issue.

of course, authors can use an actual <button>, e.g. something like

<button class="btn btn-link nav-link dropdown-toggle" href="#" id="navbarDropdownMenuLink" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">Dropdown link</button>

but currently the baseline for the text in the links and the text in the button don't always correctly line up.