downgrade NativeViewHierarchyOptimizer deprecation from ERROR to WARNING to fix template app build (#55702)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/55702

Changelog: [internal]

Downgrading from `ERROR` to `WARNING` allows existing references to compile while still signaling that the class is deprecated and should be migrated away from.

Reviewed By: alanleedev

Differential Revision: D94095560

fbshipit-source-id: dc203b88af129c7505269f89f039db99403f3e4f