WIP: update core and next packages to next15 and react19.

[jacobdubail]

TODO:

• build currently fails due to mismatching types in vite-react project
• some tests fail where they use cookies() and headers() directly instead of awaiting them.
• ...

Description of the Change

Upgrades to Next 15 and React 19.

How to test the Change

TBD.

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added Critical Flows, Test Cases, and/or End-to-End Tests to cover my change.
• All new and existing tests pass.

[changeset-bot]

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Latest commit: 502e853

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Name	Status	Preview	Comments	Updated (UTC)
headstartwp-app-router	❌ Failed (Inspect)			Dec 4, 2024 2:23am
headstarwp	❌ Failed (Inspect)			Dec 4, 2024 2:23am