[Epic] React 19 support

> [!Note]
> This work isn't yet prioritized. We're hoping to get to it in the next 6 months (~Nov 2025), but can't promise anything at this moment.

# Summary

We want EUI to support React 19 to utilize its concurrent rendering features and enable our consumers to upgrade.

# Current state

Currently, EUI officially supports React 16 through 18. We have a few legacy components that don't work well with `` and were planning to fix them for quite a while now. We need to refactor these legacy components first before we're able to add React 19 support. This work is not part of this epic and is covered by https://github.com/elastic/eui/issues/7774.

# Definition of Done

* EUI is fully compatible with React 19 environments
* EUI's documentation is updated to reflect the added support for React 19
* Local and CI test environments are updated to test against React 19