update Stylelint to v16.9.0

[romainmenke]

Hi 👋

We started monitoring prominent Stylelint packages to ensure that we don't accidentally break things when rolling out new features.

We noticed that @shopify/stylelint-plugin was failing these tests for two reasons.

---

1. It uses --formatter=github to have stable lint output that is easily testable.
   This formatter was deprecated and the deprecation warning alone was causing the tests to fail.
   I've switched this to --formatter=json which should also give stable outcomes.

2. There were also various fixes in Stylelint rules to the reported start and end positions.
   This changed the expected outcome of one of the tests.

[BPScott]

Thanks @romainmenke, I've shipped an adjusted version of this in #436, based upon how I handled the github formatter deprecation in prettier/stylelint-prettier#367

[romainmenke]

Cool! thanks 🙇