rubocop: Set TargetRubyVersion to 3.1 & disable all cops that fail

[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?

---

• Supersedes RuboCop autofixes for Ruby 3.1 #16337 (for now).
• As discussed in RuboCop autofixes for Ruby 3.1 #16337 (comment), then we won't have the chicken/egg problem.

[MikeMcQuaid]

I think these comments still need to stay around but perhaps in a different format otherwise I worry they will get "lost" when changing Style/MutableConstant back. @issyl0 can you do this in a follow-up PR? Thanks!

[Bo98]

Given this T.type_alias thing is unfixable on our side, it might be worth creating a cop that injects into Style/MutableConstant and allows T.type_alias.

Something similar to how https://github.com/Shopify/rubocop-sorbet/blob/main/lib/rubocop/cop/sorbet/mutable_constant_sorbet_aware_behaviour.rb works for T.let. Unlike T.type_alias, T.let can be frozen, just not in a way RuboCop understands. What we'd do differently for T.type_alias is we'd probably do a return if t_type_alias in on_assignment rather than passthrough.

[MikeMcQuaid]

Thanks @issyl0!