Setting up Ruby 3 on EYC Stack v6

Ruby 3 is currently supported on the stable-v6 through the following 2 actions:

1. Setting the EY_RUBY_VERSION environment variable as detailed on https://github.com/engineyard/ey-cookbooks-stable-v6/blob/next-release/EnvironmentVariables.md
2. Making use of engineyard-serverside 3.0.3 to deploy, which can be selected via the "more opts" link next to the deploy button in the UI, or via the -S switch on the ey-core deploy command, after upgrading to the latest version