Which issue, if any, is this issue related to?

Closes #6961.

Is there anything in the PR that needs further explanation?

I apologize for how large this change is 😅 I'm not sure if this is easier to review (since it is clean + accomplishes it in one go), or if we'd prefer one that splits them up piecemeal (e.g. into sections) - let me know what we'd prefer!

I'll quickly explain the methodology for this PR:

1. remove each lib/rules/* for each deprecated test
2. remove each (now invalid) path from lib/rules/index.js
3. fix all broken tests, which fall into one of several categories:
   • tests that use a deprecated rule as an arbitrary rule; to resolve this, I just picked a different rule (usually color-named for color-hex-case and number-max-precision for indentation)
     • one of these is about a custom plugin, so I've rewritten the plugin
   • tests that are about deprecated rules; I added a .skip (have left comments in the review)
   • fs tests that use deprecated rules; I removed these from the config, then updated the snapshot.
     • the zen garden one doesn't work "out of the box" anymore, i.e. the snapshot has nontrivially changed because of indentation/spacing rules
   • indentation-specific behaviour (it being last); I deleted these!
4. remove all documentation related to removed rules (mostly indentation)
5. remove all unused utilities (via code coverage, not manual inspection)