[EuiAccordion] Remove will-change CSS

[cee-chen]

Summary

EuiAccordion's will-change CSS creates a stacking context which causes nested positioned components to behave unexpectedly, e.g. fullscreen mode in EuiDataGrid:

We're opting removing this CSS for several reasons:

• [EuiAccordion] Replace translateZ with will-change #5806:
  • "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"
• It looks like translateZ was first added by 5 years ago in a relatively unrelated component PR, so we're guessing the addition this was an unnecessary pre-optimization that was popular in the frontend world at the time
• Mozilla/MDN has since recommended against using will-change:
  • "Warning: will-change is intended to be used as a last resort, in order to try to deal with existing performance problems. It should not be used to anticipate performance problems"

QA

• Confirm that https://eui.elastic.co/pr_6235/#/tabular-content/data-grid full screen mode works correctly and does not look broken
• Test https://eui.elastic.co/pr_6235/#/layout/accordion performance in Chrome, Safari, Edge, and Firefox, and confirm no significant degradation of performance

Checklist

• A changelog entry exists and is marked appropriately
• Revert [REVERT ME] commit

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_6235/

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_6235/

[chandlerprall]

Change LGTM, tested the accordion examples across browsers and don't see any difference

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_6235/