Remove misleading "toggle" phrase from Olivero's wide search form disclosure button

Split off from #3223332: Olivero primary search button should be initialized with aria-expanded="false" by JS and similar to #3228140: Remove misleading "toggle" phrase from Olivero's main-menu disclosure button.

From @andrewmacpherson

Meanwhile, another little nitpick with the button. The accessible name is a bit poor: aria-label="Toggle Search Form".

There's no need for the word "Toggle" here, because...

1. It isn't needed to convey the open/close behaviour. The mere presence of an aria-expanded attribute (whether true or false) is sufficient to convey the behaviour. A screen reader will say "button, collapsed" or "button, expanded" which conveys that it is expand/collapse-ible.
2. "Toggle" can actually be misleading, because "Toggle button" is how some screen readers (in the English locale, at least) announce <button aria-pressed="true|false">. Using "toggle" as part of the name for a disclosure button may be sending mixed message about how it behaves. I think we covered this already? Perhaps it was in an issue about the sub-navigation buttons.

Let's improve it here, under the overall scope of WCAG "Name, role, value".

TODO: Remove the word "toggle" from the aria-label - aria-label="Search Form". Then screen readers will typically say "Search Form, button, collapsed". A user will expect a search form to be revealed by pressing the button.