[REQUEST] Bring back support of Ruby 2.3, 2.4 and 2.5

[arielj]

Description

This PR dropped the support of Ruby 2.5 and older https://github.com/fastruby/next_rails/pull/121, but after some discussions we think the gem should support at Ruby 2.3 and newer.

Possible Implementation

Currently, there's no limitation of the required ruby version by our dependencies and the app was already compatible, so I think that reverting the CI changes to re-add 2.3 2.4and2.5` should be enough, I imagine tests should still pass.

If the rainbow gem turns out to be a problem, it's probably better to remove it completely to increase the support the older Ruby versions.

Resources:

The PR linked above mentions the rainbow gem and it supports ruby from version 2.3 https://rubygems.org/gems/rainbow/versions/3.1.1

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