fix(amazon-cognito-identity-js): update podspec dependency

[amhinson]

Issue #, if available:
#7747

Description of changes:

Xcode 12 will fail to build apps using amazon-cognito-identity-js when no other module in the project has an explicit dependency on React-Core. Based on the guidance from facebook/react-native#29633 (comment), we need to update our dependency in the podspec from React to React-Core, since React is essentially an umbrella dependency.

I have tested this locally with verdaccio to verify the update does NOT cause any breaking changes.

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

Merging #7873 (f8db481) into main (c43d4d0) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #7873   +/-   ##
=======================================
  Coverage   74.26%   74.26%           
=======================================
  Files         215      215           
  Lines       13470    13470           
  Branches     2645     2645           
=======================================
  Hits        10004    10004           
  Misses       3268     3268           
  Partials      198      198           

Impacted Files	Coverage Δ
packages/core/src/Util/DateUtils.ts	88.23% <ø> (ø)
packages/core/src/Platform/version.ts	100.00% <100.00%> (ø)

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[Ashish-Nanda]

LGTM

[github-actions]

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