Updated metro config warning to reflect SDK 41 by default.

[EvanBacon]

Why

• In SDK 41 the default metro config should be extending expo/metro-config (@expo/metro-config in older SDK versions).

How

Updated the warning when the config is modified to reflect the new default metro.config.js:

⚠️  Skipped Metro config updates:
› Existing Metro config found; not overwriting.
› You will need to extend the default @expo/metro-config in your Metro config.
  Learn more: https://docs.expo.io/guides/customizing-metro

• Shortened configuration to config to match some other places in the CLI
• Make the warning message a bit more deliberate as this is common behavior.

Test Plan

• expo prebuild -p android --no-install
• Modify the metro.config.js, repeating step 2 should log the new warning.
• Optionally you can change the project SDK version to 41.0.0