Official Rails 7 & Ruby 3.1 support, plus linting/CI updates

[smudge]

Summary

This adds an appraisal for railties v 7.0, and adds CI coverage for Ruby 2.6 through 3.1.

For Ruby 3.0+ support, I had to:

• Make more consistent use of keyword arguments for methods that expect them. (i.e. passing **args instead of args)
• Change the optional force: false argument on "change protection" method overrides so that it acts as a hashy opts = {} argument instead of a keyword argument. This ensures that a method call like update_all(foo: 'bar') will treat foo: 'bar' as the attributes argument, instead of eliding it into the keyword args.

Other Information

• I switched from CircleCI to GitHub Actions, mostly to get coverage across multiple Ruby versions, but also because it's a lot simpler for a project like this.
• Switched from rubocop-betterment to betterlint and made a bunch of linter corrections.
• I also added uncruft and addressed a couple spec-only deprecation warnings.

/domain @Betterment/journaled-owners @coreyja /no-platform

[nanda-prbot]

Needs somebody from @Betterment/journaled-owners and @coreyja to claim domain review

Use the shovel operator to claim, e.g.:

@myname << domain && platform

_{HOW TO: Claim a Review}

[matsuimp]

<<domainlgtm!