breaking: Remove fallback flow for Metro config defaults (0.73)

[huntie]

Summary

This follows up the @react-native/metro-config setup added in React Native 0.72. It removes the fallback to a replicated internal copy of Metro config defaults in cli-plugin-metro, effectively reverting #1896.

Rollout plan recap:

• 0.72: Extending @react-native/metro-config is recommended, but falls back if users have not updated their metro.config.js, logging a warning.
• 0.73 (this diff): Updating metro.config.js to the new format is required — log warning and (later on) fail.

Changelog: [Breaking] Remove fallback flow for Metro config defaults (React Native 0.73)

Test Plan

Using rn-tester configured with a locally yarn linked copy of RN CLI.

✅ Correct config logs no warning and builds successfully

✅ Incorrect config logs updated warning and fails

⬇️ FYI @cortinico @kelset, in case you have opinions for better DevX. It is marginally restrictive that we require a reference to @react-native/metro-config in the immediate metro.config.js file — perhaps if we detect this more strongly we can fail rather than warn. However I can't think of a simple way to do this — and we will have had the 0.72 release cycle for people to update their projects by this point. Addressed with more visible warning.

[thymikee]

This ties the move of the cli-plugin-metro effort to the core with v0.73, right? Maybe we should start merging those changes to a parallel branch with the plugin removal?

[huntie]

@thymikee It does — I was aiming to stack a sequence of simplification changes, including this PR, here in the CLI repo first. This is to slightly reduce the surface area that we are migrating into the RN repo (and from TypeScript into Flow 🙈).

A change like this also gets better visibility in this repo for now.

[kelset]

added a mention here reactwg/react-native-releases#64 so that we/the 73 release crew won't forget that there's BC on this angle

[huntie]

@kelset Sidenote: I'm looking to merge this change ahead of work to migrate and collapse cli-plugin-metro back into the React Native repository, with the intent being to minimise the number of files/surface area before this migration.

---

With some over-the-weekend reflection — the flow in this PR should probably hard-error (helpful in particular to pin down this error cause in CI) ➡️ update incoming.

For the user, it might be better if we allowed the entire metro.config.js file to be optional (but reinsertable, for standalone metro commands) (especially after facebook/metro#977 ships). We can look at this post-migration, since it'll be possible to refer to @react-native/metro-config in the same repo.

[huntie]

Latest push

• This check remains a warning, but is made much more visible (as lack of this config will break builds except for rare advanced usages). cc @motiz88 for wording sign off.
• Now includes the updated check from feat: Add alternative base config check to loadMetroConfig (11.x) #1990 — on main we can safely depend on the future version of @react-native/metro-config with the new detection mechanism present.

✅ Ready for review

[motiz88]

Wonder if this should be pinned (at least to a branch) so it remains accurate in the future.

[huntie]

Yeah fair. We don't have 0.73-stable yet — and the counter-risk is pointing to a stale copy. Will keep an eye on it.