Update RNTester CocoaPods to 1.8.4

[friederbluemle]

Summary

This is the second half of #26976 (the CocoaPods update did not make it to master).

@RSNara @hramos @fkgozali @PeteTheHeat - Please take a quick look and comment if there are any known issues with using the latest stable version of CocoaPods. Thanks! 🙏

It seems like the older version of CocoaPods has a bug that lowercases the spec repo url, even in the unfortunate case that the repo maintainers use upper case characters. This would essentially revert what was fixed in #26976.

Changelog

[General] [Changed] - Update RNTester CocoaPods to 1.8.4

Test Plan

cd RNTester
bundle install
bundle exec pod install

[mdvacca]

Importing to review and land internally

[facebook-github-bot]

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[react-native-bot]

This pull request was successfully merged by @friederbluemle in 30197c3.

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