Rails > 6 and Ruby > 3 support

[sosolidkk]

We extensively rely on the ar-octopus gem in various projects within our company. However, due to them being on an older version of Rails that lacks the matured functionalities of the new multiple database support, we are unable to leverage it. Nevertheless, this gem remains crucial for effectively managing our shards and facilitating database replica reading.

To ensure compatibility with Rails version > 6 and Ruby > 3, modifications were required for our application to function correctly. Unfortunately, some of these changes may break existing applications utilizing this gem. However, I think we can collaborate to review these modifications and find a way to incorporate them into the gem, making it adaptable to everyone's use cases.

[sosolidkk]

Hey @ylecuyer and @kammradt. Since @SixiS put up #578 and it's way cleaner and more focused on the changes that we really need to put up the upgrade and support for Rails 6 and Ruby 3, feel free to close this PR. I'm happy enough that it served as a starting point for this upgrade to happen.