Introduce Gemfile, ruby-version (#32303)

Summary:
Implement par of the discussion https://github.com/react-native-community/discussions-and-proposals/discussions/411, except the `.nvmrc` part, this includes:
 - Setting `.ruby-version` in the main project and also `template/`
 - Fixing the CocoaPods version with a project-level `Gemfile` and also `template/Gemfile`
 - Using all `pod` executions from `bundle exec pod`, using the determined version
 - Script to manage and update the ruby version

## Changelog

[iOS] [Added] - Gemfile with CocoaPods 1.11 and ruby-version (2.7.4)

Pull Request resolved: https://github.com/facebook/react-native/pull/32303

Test Plan: Build for iOS and run all CircleCI tests to see if nothing changed

Reviewed By: RSNara

Differential Revision: D31344686

Pulled By: fkgozali

fbshipit-source-id: 25c63131ca9b16d3cf6341019548e0d63bdcaefe