rubocop: Use Sorbet/StrictSigil as it's better than comments

[issyl0]

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• Part of Use Sorbet typed: strict in all (non-package) files in Homebrew organisation #17297.
• Previously I thought that comments were fine to discourage people from wasting their time trying to bump things that used undef that Sorbet didn't support. But RuboCop is better at this since it'll complain if the comments are unnecessary.
• Suggested in ci: Encourage Sorbet # typed: strict in new code #18018 (comment).
• I've gone for a mixture of rubocop:disable for the files that can't be typed: strict (use of undef, required before everything else, etc) and rubocop:todo for everything else that should be tried to make strictly typed. There's no functional difference between the two as rubocop:todo is rubocop:disable with a different name.
• And I entirely disabled the cop for the docs/ directory since typed: strict isn't going to gain us anything for some Markdown linting config files.

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• This means that now it's easier to track what needs to be done rather than relying on checklists of files in our big Sorbet issue:

$ git grep 'typed: true # rubocop:todo Sorbet/StrictSigil' | wc -l
    268

• And this is confirmed working for new files:

$ git status
On branch use-rubocop-for-sorbet-strict-sigils
Untracked files:
  (use "git add <file>..." to include in what will be committed)
        Library/Homebrew/bad.rb
        Library/Homebrew/good.rb

nothing added to commit but untracked files present (use "git add" to track)

$ brew style
Offenses:

bad.rb:1:1: C: Sorbet/StrictSigil: Sorbet sigil should be at least strict got true.
# typed: true
^^^^^^^^^^^^^

1340 files inspected, 1 offense detected

[MikeMcQuaid]

Thanks @issyl0! Unfortunately there's now a bunch of conflicts but good to self-merge once those are resolved.