rubocop: adjust RuboCop metrics checks.

[MikeMcQuaid]

We want to allow this class to grow larger still while still enforcing this on other classes.

• 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 tests with your changes locally?

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[maxim-belkin]

This PR goes a little bit beyond changing class length for formula.rb so it might be a good idea to
a) rename it accordingly ("revisit style guide" or something like that) and
b) explain changes made and provide justifications (either as comments in the file or in the commit message).

[MikeMcQuaid]

a) rename it accordingly ("revisit style guide" or something like that) and

done!

[Rylan12]

Thanks!