Update @reduxjs/toolkit to v2 (with opt-in and v1 fallback)

[walterra]

Summary

This PR replaces renovate PRs #206187 and #235548.

Fixes #239863.

Redux Toolkit V2 includes some breaking changes that will be quite an effort to fix in just one PR. Overall the use of redux and RTK is quite fragmented! Some parts use legacy redux style and might bring in other packages for selectors and alternatives to redux-thunk. Luckily there are more modern usages of RTK too.

Some new features/breaking changes:

• createSlice comes with new features for inline selectors and thunks
• combineSlices API with slice reducer injection for code-splitting, this could be quite useful to pick up for Kibana
• V2 required to work with React v18 concurrent mode!
• Redux Toolkit 2.0 is accompanied by major versions of all the other Redux packages: Redux core 5.0, React-Redux 9.0, Reselect 5.0, and Redux Thunk 3.0.
• React-Redux 9.0 makes use of React 18’s native useSyncExternalStore to support concurrent mode.

Some of Kibana’s code base uses legacy redux style and that code might also be in maintenance mode with the aim to remove it at some point. We’d like to avoid spending lots of time updating that code base to use Redux Toolkit. Considering that, it might be worth enabling support for both RTK V1 and V2 for the time being. This would also allow teams to migrate gradually like we are able to do with React V18 concurrent mode.

This PR tries to do the following: Keep RTK V1 for the time being and allow to opt in to migrate to V2. This will have good developer experience for teams, since everyone can update their code base with separate PRs. The drawback is that we’d have to maintain packages aliases for RTK and all accompanying packages mentioned above until all legacy redux code is either removed or migrated. To have both versions will add 20-25Kbyte bundle size.

Checklist

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• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• Documentation was added for features that require explanation or tutorials
• Unit or functional tests were updated or added to match the most common scenarios
• If a plugin configuration key changed, check if it needs to be allowlisted in the cloud and added to the docker list
• This was checked for breaking HTTP API changes, and any breaking changes have been approved by the breaking-change committee. The release_note:breaking label should be applied in these situations.
• Flaky Test Runner was used on any tests changed
• The PR description includes the appropriate Release Notes section, and the correct release_note:* label is applied per the guidelines
• Review the backport guidelines and apply applicable backport:* labels.

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