RNTester: Fixes crash when app back to background (#48385)

Summary:
https://github.com/facebook/react-native/issues/48376 removed `applicationDidEnterBackground` from `RCTAppDelegate` but RNTester called it, leads to crash. cc cipolleschi can you please help to review?

## Changelog:

[INTERNAL] [FIXED] - RNTester: Fixes crash when app back to background

Pull Request resolved: https://github.com/facebook/react-native/pull/48385

Test Plan: RNTester iOS back to background not crash.

Reviewed By: cipolleschi

Differential Revision: D67657449

Pulled By: philIip

fbshipit-source-id: e6d806b2677050fa2faa273a7468055d9d21c2a3