[A11y][Fastpass] Add role attribute where aria-label attribute is used

[keylime-unicorn]

Added the role="alert" attribute to the warning message for the third party package managers.

Addresses #9319

[advay26]

Why do we need this b tag here at all? It doesn't seem to add anything to the display, and the surrounding div can be given the same 'aria-label' attribute, plus it already has the role="alert" attribute. Should we just remove the b tag completely?

[keylime-unicorn]

@advay26 @drewgillies It's not supposed to add anything to the display at all. Basically, we ran into an issue a few months ago where we were told that the warning icon should alert the user that the following text is a warning (essentially just read out "warning" before reading out the text). The problem is that the only way to get this to happen while still leaving warning icon itself hidden from screen readers is to add a separate element that has no visual aspect and simply makes the screen reader read out the "warning" text.