Upgrade stylelint config

[shawnbot]

This PR is a placeholder for upgrading to stylelint-config-primer@6.0.0 once primer/stylelint-config#27 is ready. For now, let's see if the canary release of primer/stylelint-config#25 plays nicely.

[shawnbot]

Oof, well this was kind of tricky. My assumption in primer/stylelint-config#25 was that even with @primer/css as a peer dependency, it (Node) would still be able to resolve everything when run within that package. This was a bad assumption, so what I ended up doing was disabling both the primer/selector-no-utility and selector/no-override rules in this repo's stylelint config.

The result is that we still get all of the syntax and formatting rules, but we don't enforce "duplicate" selectors or classnames here because this is the source of those values.

[simurai]

Looks good to me. 😝

I assume we need to add these rules to .stylelintrc.json because primer/css should be the only exception.