Fix findDOMNode deprecation warning in slideshow widget

[rjwills28]

This warning is currently thrown when running the tests:

Warning: findDOMNode is deprecated and will be removed in the next major release. Instead, add a ref directly to the element you want to reference. Learn more about using refs safely here: https://reactjs.org/link/strict-mode-find-node
    at Transition (/cs-web-lib/node_modules/react-transition-group/cjs/Transition.js:135:30)
    at CSSTransition (/cs-web-lib/node_modules/react-transition-group/cjs/CSSTransition.js:120:35)
    at SwitchTransition (/cs-web-lib/node_modules/react-transition-group/cjs/SwitchTransition.js:159:35)
    at SwitchableWidget (/cs-web-lib/src/ui/widgets/Slideshow/slideshow.tsx:49:25)
    at div
    at div
    at SlideshowComponent (/cs-web-lib/src/ui/widgets/Slideshow/slideshow.tsx:86:61)

I've followed the advice to pass a ref directly into the element and CSSTransition component. Also see discussion here with a similar suggestion/implementation: reactjs/react-transition-group#668.

Beyond this, I'm actually not sure where the slideshow widget is used? Maybe it can actually be removed...

[abigailalexander]

Can confirm this fixes the warning.
I'm also not sure what the use of the Slideshow widget is, as it isn't part of css/phoebus. I agree we should look into whether it could be removed in future