Copy React vendor scripts from Gutenberg, update version number

[jsnajdr]

Gutenberg started building its own bundles for the react, react-dom and react-jsx-runtime vendor scripts. Previously we've been copying React's own UMD bundles shipped as part of their NPM packages, but React 19 is going to remove these UMD bundles, and we are preparing for the migration by abandoning the UMD bundles in advance.

Instead of copying the UMD files, we'll copy them from the Gutenberg build.
We'll also update the version strings of the packages by appending a -wp suffix, so that we can differentiate them from previous versions. Even though it's still the same React 18.3.1, the build method has changed.

Gutenberg started building its own bundles for the react, react-dom and react-jsx-runtime vendor scripts. Previously we've been copying React's own UMD bundles shipped as part of their NPM packages, but React 19 is going to remove these UMD bundles, and we are preparing for the migration by abandoning the UMD bundles in advance.

Instead of copying the UMD files, we copy them from the Gutenberg build.

We also update the version strings of the packages by appending a -wp suffix, so that we can differentiate them from previous versions. Even though it's still the same React 18.3.1, the build method has changed.

Trac ticket: https://core.trac.wordpress.org/ticket/64958
Backports Gutenberg PR: ​https://github.com/WordPress/gutenberg/pull/74343 and ​https://github.com/WordPress/gutenberg/pull/76811

## Use of AI Tools

Autocomplete with Cursor and Opus 4.6.

[desrosj]

@jsnajdr Is this a requirement for the 7.0 release? If not, we should hold off on merging this at least until after branching, preferably after 7.0 is released just to give it some proper testing and confirmation given the volume of changes to the build script this cycle.

[jsnajdr]

Waiting for 7.1 is fine, because 7.0 is not broken in any way. The corresponding Gutenberg fixes are urgent, because the equivalent Gutenberg functionality is broken, but the backport is not.

We also update the version strings of the packages by appending a -wp suffix, so that we can differentiate them from previous versions. Even though it's still the same React 18.3.1, the build method has changed.

We should probably get wider feedback on this new suffix. IIRC, WP always used version numbers for scripts, but vendors might be excluded from this policy.

cc @desrosj, @peterwilsoncc

Noting that I've seen this and it's on my list, but there are still build script bugs that need to be ironed out for 7.0. So I'm prioritizing those first.