Docs: Mark React Native mobile editor as unmaintained on trunk after React 19 upgrade

[tyxla]

What?

Documents the React Native mobile editor's current status on trunk following the upgrade to React 19 (#61521) and points contributors at the right branching point for any fixes to the existing Gutenberg Mobile build.

Updated files:

• docs/contributors/code/react-native/README.md — adds a Project status section as the canonical source for the current state.
• docs/contributors/code/react-native/getting-started-react-native.md — adds a heads-up at the top, linking back to the project status section.
• packages/react-native-editor/README.md, packages/react-native-bridge/README.md, packages/react-native-aztec/README.md — adds a short status callout in each package README.
• CONTRIBUTING.md — replaces the "mirror .native.js changes" directive with a note that it's no longer required on trunk, and points RN fixes at the appropriate release tag.

Why?

From the React 19 tracking issue (#71336):

While breaking Gutenberg Mobile on trunk doesn't seem to be an ideal next step, it's a necessary one. The WP Mobile project is actively exploring paths forward for the block editor in the mobile apps, aiming to reduce complexity and improve alignment with the Gutenberg web project. There is currently little to no active work on the Gutenberg Mobile project. Therefore, to avoid encumbering the web project and its surrounding community, we must upgrade React and accept that doing so introduces technical debt for Gutenberg Mobile.

This PR addresses the still-unchecked task on that tracking issue:

• Update documentation to make it clear that the RN implementation is paused and may be broken in trunk.

If a fix is needed against the existing Gutenberg Mobile build, the docs now direct contributors to branch from tag rnmobile/1.121.0 (commit e63b8b8, #63744) — the latest React Native release before the React 19 upgrade — and cut a minor release from there.

How?

Docs-only — no code or build changes. Each notice references PR #61521 and links back to the central Project status section so any entry point surfaces the same information.

Testing Instructions

• Read through the updated files and confirm the wording is accurate.
• Verify the links to Upgrade to React 19 #61521, Mobile Release v1.121.0 #63744, Upgrading to React 19 #71336, the rnmobile/1.121.0 tag, and the e63b8b8 commit resolve correctly.
• Confirm the in-repo links (e.g. from getting-started-react-native.md to the project status section) work.

Testing Instructions for Keyboard

N/A — docs only.

Screenshots or screencast

N/A — docs only.

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[Mamaduka]

Makes sense, thanks @tyxla!

Let's also ping @WordPress/native-mobile folks.