fix(ios, podspec): depend on React-Core instead of React

[mrousavy]

Description

On Xcode 12 some wild build errors occur when trying to build a React Native app. This is because the 'React' pod is only an umbrella pod. See: facebook/react-native#29633

Related issues

See: facebook/react-native#29633

Release Summary

• Uses 'React-Core' instead of 'React' pod in podspecs

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
  • Yes
• My change supports the following platforms;
  • Android
  • iOS
• My change includes tests;
  • e2e tests added or updated in packages/\*\*/e2e
  • jest tests added or updated in packages/\*\*/__tests__
• I have updated TypeScript types that are affected by my change.
• This is a breaking change;
  • Yes for React < 0.60 (because of autolinking)
  • No

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[mikehardy]

Thanks for this and I appreciate the collaboration over on the related issue since I'm having trouble reproducing it myself.

[mikehardy]

For anyone else that needs this but is having problem upgrading to current because of for instance #4229 - you may depend on the patches we generate for every PR https://github.com/invertase/react-native-firebase/actions/runs/259924803 - applied via patch-package they can be used in place of updating, as a temporary measure while root causes are resolved