EuiButtonToggle/useRenderToText: prevent state update on unmounted component

[thompsongl]

Summary

Fixes #2736.

• Store the DocumentFragment reference in a mutable useRef object so it's (synchronously) modifiable and available for the lifecycle duration.
• The ref callback from useInnerText is a useState setter, so we need to prevent invocations after unmount (hostNode == null).
• Remove requestAnimationFrame tick as there are no more state side-effects during the unmount process.

Simple testing method with local EUI docs:

• "Toggle buttons" section
• Change from "Demo" to "Props" tab
• You'll see a (soon to be fixed) EuiCodeBlockImpl warning, but no state update warning

Checklist

- [ ] Check against all themes for compatibility in both light and dark modes
- [ ] Checked in mobile

• Checked in IE11 and Firefox

- [ ] Props have proper autodocs
- [ ] Added documentation examples
- [ ] Added or updated jest tests

• Checked for breaking changes and labeled appropriately

- [ ] Checked for accessibility including keyboard-only and screenreader modes

• A changelog entry exists and is marked appropriately

[andrew-goldstein]

👍 it's nice that the RAF trick is no longer required

[andrew-goldstein]

Thanks @thompsongl for the quick turnaround on this for 7.6! 🙏
LGTM