chore: bump to React 19

[barjin]

Following the changes from https://github.com/apify/docs-search-modal/pull/12, this PR bumps Docusaurus and React versions in both the main docs project and the @apify/docs-theme package.

Before undrafting, we need to ensure all the subdocs (Apify SDK, CLI, Clients) are ready for the React 19 switch.