Bump Xcode to 12.1.0 and use Gemfile CocoaPods version during pod install

[hramos]

Summary:
Upgrade the version of Xcode used in Circle CI to Xcode 12.1.0, and target iOS 14.1 in tests.

Reference:

• Circle CI Xcode 12.1 container manifest (new version):
  https://circle-macos-docs.s3.amazonaws.com/image-manifest/v3985/index.html
• Circle CI Xcode 11.6.0 container manifest (previous version): https://circle-macos-docs.s3.amazonaws.com/image-manifest/v3299/index.html

Source: https://circleci.com/docs/2.0/testing-ios/#supported-xcode-versions

The default version of CocoaPods used in the Xcode 12.1 container image has changed. Use bundle exec with pod install to ensure we use the same version of CocoaPods as prescribed in the Gemfile.

Changelog:
[iOS][Changed] - Update iOS project template for Xcode 12.1.0
[Internal] Bump Xcode version used for iOS tests.

Differential Revision: D24545010

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D24545010

[pull-bot]

	Messages
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Generated by 🚫 dangerJS against 5db3a52

[analysis-bot]

Platform	Engine	Arch	Size (bytes)	Diff
ios	-	universal	n/a	--

Base commit: 36b0f7d

[hramos]

This is an export of an internal diff. The internal diff has been approved, but we're blocked until Xcode 12.1.0 has finished rolling out to our internal CI.

[react-native-bot]

This pull request was successfully merged by @hramos in 538446e.

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