Remove .ruby-version and update Gemfile to support multiple versions of Ruby (#36281)

Summary:
This Change remove the need to have a specific version of Ruby installed. We are now supporting a wider range of Ruby versions, starting from Ruby 2.6.10 (the ruby installed on MacOS by default).
We are still using a Gemfile to control the version of cocoapods that needs to be installed.

## Changelog

[IOS] [CHANGED] - Remove `.ruby-version` and update Gemfile to support a wider range of Ruby versions

Pull Request resolved: https://github.com/facebook/react-native/pull/36281

Test Plan:
- Tested locally on RNTester and an app from template, switch across different versions of Ruby
- CircleCI is green on the commit

Reviewed By: cortinico

Differential Revision: D43567660

Pulled By: cipolleschi

fbshipit-source-id: e7edfe5806a898a83ba39cb58b1318ebde56a57c