chore: update RNTester Cocoapods to 1.13 (#41248)

Summary:
This PR updates the internal version of cocoapods to 1.13, template already uses this version. I've also removed the root folder Gemfile as it's not necessary anymore.

## Changelog:

[INTERNAL] [CHANGED] - Update RNTester Cocoapods to 1.13

Pull Request resolved: https://github.com/facebook/react-native/pull/41248

Test Plan:
Check if cocoapods installs correctly by running:

1. `bundle install`
2. `bundle exec pod install`

Reviewed By: dmytrorykun

Differential Revision: D50972135

Pulled By: cipolleschi

fbshipit-source-id: b7d6a4671e641b7b8f50242a3374f623e023daf4