Read `.ruby-version` file in `Gemfile` (#35410)

Summary:
When updating the Ruby version, 2 files are always needed to be updated (`.ruby-version` and `Gemfile`). When not in sync it can lead to an error like `Your Ruby version is 2.7.6, but your Gemfile specified 2.7.5`.

This lessens the files that need to be updated when upgrading the Ruby version and makes it in sync always. It makes the `.ruby-version` the source of truth.

### Example 1:

<img width="481" alt="Screenshot 2022-11-20 at 13 56 08" src="https://user-images.githubusercontent.com/36528176/202888191-450ab6d0-32a4-4d37-ad82-0beb2b22fa82.png">

When upgrading from `0.70.0` to `0.71.0-rc.0`. 2 files needs to be updated when it could have been just 1.

Source: https://react-native-community.github.io/upgrade-helper/?from=0.70.0&to=0.71.0-rc.0

### Example 2:

21c8837c1264cd96f5899e33132fa764bb9c2293 updates 4 files (`.ruby-version`, `Gemfile`, `template/Gemfile`, `template/_ruby-version`) when it could have been just 2.

### Other Sources:
* https://andycroll.com/ruby/read-ruby-version-in-your-gemfile/
* https://render.com/docs/ruby-version (Heroku alternative)
* https://stackoverflow.com/a/35823132/9375533

## Changelog

[General] [Changed] - Read `.ruby-version` file in `Gemfile`

Pull Request resolved: https://github.com/facebook/react-native/pull/35410

Test Plan: Only `.ruby-version` and `template/_ruby-version` needs to be updated when upgrading Ruby version.

Reviewed By: christophpurrer, cipolleschi

Differential Revision: D41429147

Pulled By: rshest

fbshipit-source-id: 9e541a1d84aed5dca1e6f465c61bb0ba15574211