Package deprecations

[tomjn]

npm WARN deprecated @stylelint/[email protected]: Use the original unforked package instead: postcss-markdown
npm WARN deprecated [email protected]: This package has been deprecated, please use @wordpress/stylelint-config or @wordpress/scripts

There may be others, I believe we can fix this by moving from v18 to v20 of @wordpress/stylelint-config:

https://github.com/humanmade/coding-standards/blob/master/packages/stylelint-config/package.json#L30

This should eliminate some deprecated package warnings in dependent projects