Rubocop autocorrect all Ruby files, fail CI if warnings found

[PatrickF1]

I want to require everyone to properly lint their files before merging to master to reduce bugs. Ideally, we would also automatically run rubocop --autocorrect on file save (easily achievable through an editor integration) so all our code has consistent style and conventions.

So, this PR

• updates to and requires the current latest RuboCop
• removed some annoying code metric cops
• turned on a few disabled cops I found useful
• rubocop autocorrected all our Ruby files to fix our current warnings and prevent autocorrect from producing out-of-scope diffs in subsequent PRs
• updated CI to fail builds if there are warning-level offenses detected

Ideally, CI would also fail if there are autocorrectable offenses, even if not warning level, but I couldn't figure out how. I asked in rubocop/rubocop#12702 and will follow up once I get the answer.
I will update our dev docs to make it clear we should be using RuboCop.