Migrate TargetRailsVersion to the new requires_gem RuboCop API

[amomchilov]

This PR updates the TargetRailsVersion to conditionally use the requires_gem API when available (depending on the version of the base RuboCop gem). This has several advantages:

1. It resolves a circular dependancy.
   • rubocop-rails depends on rubocop (e.g. for RuboCop::Cop::Base), and that's fine, but there's also a hidden reverse dependency, where rubocop implicitly depends on rubocop-rails.
   • Now it's just one-way. rubocop-rails registers its minimum version requirements, and rubocop knows how to understand those without needing to call back into rubcop-rails.
2. It removes a "snowflake" case that's treated specially. For now we need to support both for backwards compatibility, but eventually when old versions are drop, it can just standardize on requires_gem.
3. It adds rails requirements to a Cop's gem_requirements list, just like any other.

---

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.

[amomchilov]

Hey @bbatsov @koic,

Now that rubocop/rubocop#12186 is shipped, this is ready to review/merge.

[koic]

@amomchilov This looks good to me. Can you squash your commits into one?

[amomchilov]

@koic Are you sure these should be squashed? The first commit is a correction to the test data, which is incorrect to begin with, unrelated to the changes in the second commit.

I was also apprehensive to change tests and impl in the same commit, but I can squash if you're sure

[koic]

@amomchilov Yes, due to the test not passing when using the new API, please squash the related changes into a single commit for maintenance purposes.

[amomchilov]

Done, I squashed the two together, and rebased over the latest master.

[koic]

Great! Thank you!