Update RNTester CocoaPods to 1.8.4 (facebook#27173)

Summary:
This is the second half of facebook#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 facebook#26976.

## Changelog

[General] [Changed] - Update RNTester CocoaPods to 1.8.4
Pull Request resolved: facebook#27173

Test Plan:
```
cd RNTester
bundle install
bundle exec pod install
```

Reviewed By: RSNara

Differential Revision: D18413420

Pulled By: mdvacca

fbshipit-source-id: 9c8794054f6e831a4a41ca96944767240c1462bf

Author: friederbluemle

RNTester/Gemfile

@@ -1,4 +1,4 @@
 # Gemfile
 source 'https://rubygems.org'
 
-gem 'cocoapods', '= 1.7.1'
+gem 'cocoapods', '= 1.8.4'