Add ActiveRecord 7 and Ruby 3.1 to CI

[petergoldstein]

This PR adds Ruby 3.1 and ActiveRecord 7.0 to the CI matrix in the following combinations:

1. Ruby 3.0 / ActiveRecord 7.0
2. Ruby 3.1 / ActiveRecord 6.1
3. Ruby 3.1 / ActiveRecord 7.0

With the additional changes below, the new branches all run green.

In addition to the CI configuration updates, this PR includes the following changes:

1. Explicitly adding the net-imap, net-pop, and net-smtp gems to the Rails 6.1 gemfile when Ruby is 3.1 or higher
2. Tweaking a few requires so that class loading works correctly in Rails 7

I didn't add a CHANGELOG entry because I'm not sure if the maintainers would want to treat this PR as an expansion of the officially supported Ruby and ActiveRecord versions.

---

Before submitting the PR make sure the following are checked:

• [X] The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• [X] Wrote good commit messages.
• [X] Feature branch is up-to-date with master (if not - rebase it).
• [X] Squashed related commits together.
• [ ] Added an entry to the changelog if the new code introduces user-observable changes. See changelog entry format for details.

[petergoldstein]

@tindron I've removed those lines and everything is green on my end,

[rabotyaga]

Hey @petergoldstein 👋 ! Thanks, done in https://github.com/toptal/chewy/pull/834.