Summary

Part of #4402. Removes makeCompatibleObserver fallback originally written for a time when EUI's supported browsers did not all implement the ResizeObserver API.

Note that the tests have been skipped:

The makeCompatibleObserver fallback method is no longer necessary.

This is true, except for jest environments. jsdom does not implement the ResizeObserver API, which means that jest is using the MutationObserver fallback in makeCompatibleObserver. It appears that resize-observer-polyfill often recommended for jest environments uses a similar MutationObserver fallback approach.
So, by removing our MutationObserver fallback and relying solely on ResizeObserver, we'll lose the ability to test EuiResizeObserver and useResizeObserver. This seems fine, because our supported browsers all implement the ResizeObserver API, which means our tests were not actually testing the real, intended behavior of target browsers. (#4402 (comment))

Checklist

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

• Checked in Chrome, Safari, Edge, and Firefox

- [ ] Props have proper autodocs and playground toggles

• Added documentation
• Checked Code Sandbox works for the any docs 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