RuboCop autofixes for Ruby 3.1

[issyl0]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

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• As noted in Slack, bumping RuboCop's target Ruby version to the one we actually use now reveals a bunch of improvements, all autofixable.
• This is draft because I have to change trains now - I'll return to this later.

[MikeMcQuaid]

Thanks @issyl0! If you can make this PR non-draft after CI is 🟢: let's get it merged ASAP. This is going to be much better/easier to handle with lots of smaller PRs (maybe even one commit per PR) than one big PR that grows and stays open for a while.

[Bo98]

TargetRubyVersion is needed as it controls whether RuboCop thinks this new syntax is valid.

It will also make this a lot less trivial as you will then need to convert every tap and merge them all at the same time. Half why I dumped a Gist link and walked away :-)

[issyl0]

Oh, but 2.6 and 3.1 disagree with each other. Joy.

[Bo98]

The easiest way is likely shipping TargetRubyVersion first in its own PR, but with all new cops disabled. And then have a second PR (this one) where you enable the cops. That way the changes can be merged in taps without having a chicken/egg problem.

[issyl0]

Yeah, I realised this is bigger than I thought. We can do it though! Somehow.

[issyl0]

Good idea. I'll do that soon.