[Future task] Remove @react-native/metro-config warning from `react-native start`

[huntie]

Context

• RFC: Add @react-native/metro-config package react-native#36502
• breaking: Remove fallback flow for Metro config defaults (0.73) react-native-community/cli#1972

In React Native 0.73, we're shipping a prominent warning in the CLI when we can't detect that the user's Metro config extends @react-native/metro-config, which contains the base Metro config to work with React Native.

In the vast majority of projects, not extending @react-native/metro-config will lead to a failure to build, often with unclear errors. Therefore we are strongly recommending extending it even for highly custom setups.

However, we anticipate there will advanced users that fully define a compatible Metro config without the base package. For these users, this warning will be redundant. We anticipate that this will be a small enough user subset that we are not providing an official opt-out — however we are committing to removing this warning in general.

This task

Once we have very high confidence that all users have migrated to the new config setup, we will drop the CLI warning.

Potential related tasks:

• To help with issue intake once we've removed this warning, we could add metadata on whether the base config is loaded, and at what version, via react-native doctor.