Context: #1079.

Context: #1074, #1076. I think the problem is likely with `@use "sass:math";`; the stock pages image doesn't contain an up-to-date enough version of SASS. I've instead replaced just that instance with a runtime `calc()` operation, which *should* get optimized away by the compiler (see: [SASS docs](https://sass-lang.com/documentation/breaking-changes/slash-div#transition-period)).

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Original PR body:

> @pdmosses noticed that we have deprecation warnings on some of our SASS code. After testing locally, all of them have to do with using / as division in SASS, which is [deprecated](https://sass-lang.com/documentation/breaking-changes/slash-div) (since there's some lexical ambiguity).
> 
> SASS has a nifty [migrator tool](https://github.com/sass/migrator). I used the migrator piecewise on each deprecation warning (since the global usage fails on some liquid code). Upon manual inspection, I think there are no false positives. Running bundle exec jekyll serve after a fresh install and bundle update no longer emits any warnings.

@pdmosses noticed that we have deprecation warnings on some of our SASS code. After testing locally, all of them have to do with using `/` as division in SASS, which is [deprecated](https://sass-lang.com/documentation/breaking-changes/slash-div) (since there's some lexical ambiguity). 

SASS has a nifty [migrator tool](https://github.com/sass/migrator). I used the migrator piecewise on each deprecation warning (since the global usage fails on some liquid code). Upon manual inspection, I think there are no false positives. Running `bundle exec jekyll serve` after a fresh install and `bundle update` no longer emits any warnings.

Closes #1073; blocked by #1072 (CI failure).

This is a catch-all PR that modernizes and updates our Stylelint config, and resolves all open issues. This is a pretty big change - so I want to update all of our related dependencies in lockstep.

In particular, this PR

- [x] updates stylelint to `v14`
- [x] adds in the standard stylelint config for SCSS (`stylelint-config-standard-scss`)
- [x] swaps out `stylelint-config-prettier` for `stylelint-config-prettier-scss`
- [x] ~~properly update `@primer`-related plugins:~~ completely remove `primer` from our configuration
- [x] autofix, manually resolve, or disable all newly-introduced lint errors; **I've avoided manually resolving errors that would be a behavioural change**
- [x] re-runs `npm run format`

See the "next steps" section on some extra thoughts on disabling errors.

(implicitly, I'm also using node 16/the new package-lock format).

### disabling rules and next steps

I've introduced several new disabled rules. Let me quickly explain what's going on; there are two categories of rules I've disabled:

1. rules that were temporary disables; they were frequent enough that I couldn't manually resolve them, but should be simple. **I plan on opening issues to re-enable each of these rules**, just after this PR
    - `declaration-block-no-redundant-longhand-properties`: this is just tedious and error-prone
    - `no-descending-specificity`: this one is tricky since it could have impacts on the cascade (though that seems unlikely)
    - `scss/no-global-function-names`: I think we need to [import map and then use `map.get`](https://stackoverflow.com/questions/64210390/sass-map-get-doesnt-work-map-get-does-what-gives), but I'll leave this as out of scope for now
2. rules that are long-term disables; due to the SASS-based nature of our theme, I think we'll keep these in limbo
    - `alpha-value-notation` causes problems with SASS using the `modern` syntax - literals like `50%` are not properly interpolated, and they cause formatting issues on the site
    - `color-function-notation` also causes problems with SASS, but in this case the `modern` syntax breaks SASS compilation; we're not alone (see this [SO post](https://stackoverflow.com/questions/71805735/error-function-rgb-is-missing-argument-green-in-sass)). 

In addition, we have many inline `stylelint-disable` comments. I'd open a separate issue to audit them, especially since I think some disables are unnecessary.

### on Primer 

**note: there hasn't been much other discussion, so I'm going to remove primer's stylelint config.**

If I do add `@primer/stylelint-config`, I get *a ton* of errors about now using `@primer`'s in-built SCSS variables. I imagine that we probably won't want to use these presets (though I could be wrong). In that case, I think we could either:

1. disable all of those rules
4. not use `@primer/stylelint-config`, since we're not actually using primer, and shift back to the standard SCSS config provided by Stylelint

~~Any thoughts here? I also don't have the original context as to why we do use the primer rules, perhaps @pmarsceill can chime in?~~