Add backface-visibility to .hover-grow

[brandonrosage]

Fixes https://github.com/github/education-web/issues/1672

This PR adds one line of CSS to the .hover-grow utility class: backface-visibility: hidden;

That line addresses a visual bug where any images contained within the element would flicker or distort briefly during the animation.

/cc @primer/ds-core

[broccolini]

@brandonrosage @sophshep Do you need us to ship this quickly? Just checking as we didn't have a release planned next week but happy to do a patch release if needed.

[sophshep]

I'd categorize this as a "nice-to-have" since it's adding polish vs. fixing something that's broken. I think if you've got a release in the next couple of weeks that would be fine. Thanks for checking though @broccolini!