[EuiAccordion] Replace translateZ with will-change

[thompsongl]

Summary

Closes #3548

Using translateZ causes any position: fixed; content to render in the wrong location.
My guess is that transform: translatez(0); was used to either 1) force hardware acceleration of transition effects or 2) resolve rendering a bug in an old/unsupported browser.
If the latter, we're no longer affected. If the former, we can achieve hardware acceleration with will-change: opacity;; the other values are nice-to-haves in this case.

It's also very possible that we don't need to engage the GPU at all and even transform: translatez(0); was a preoptimization. In that case, we can remove will-change, too, but the current method will render more similarly to using translateZ.

Screen.Recording.2022-04-15.at.2.37.32.PM.mov

Checklist

• Checked in Chrome, Safari, Edge, and Firefox
• Checked for breaking changes and labeled appropriately
• A changelog entry exists and is marked appropriately

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_5806/

[elizabetdev]

LGTM! 🎉

Tested locally in Chrome, Safari, Firefox, and Edge.