Infrastructure: Add @csstools/stylelint-formatter-github as Stylelint's GitHub formatter

[howard-e]

Stylelint's GitHub formatter was deprecated in v16.8.0. Running npx stylelint "**/*.css" -f github from the lint-css workflow now gives the following error (currently being thrown in #2991):

[stylelint:004] DeprecationWarning: "github" formatter is deprecated
Please use "stylelint-actions-formatters" instead.

This PR uses --custom-formatter @csstools/stylelint-formatter-github, which should be a drop in replacement for -f github

[howard-e]

cc @nschonni @jongund

[jongund]

@howard-e
What does this mean for me as an author?
I get alot of CSS linting errors in the high contrast pull request.
What is the best way to fix them?

[howard-e]

@howard-e What does this mean for me as an author?

Looking again at your latest build, this error may be more of a warning instead of forcing a process exit so maybe nothing actually! If I'm wrong on that, you could approve and merge this work into main then merge main into your branch.

I get alot of CSS linting errors in the high contrast pull request. What is the best way to fix them?

Running npm run fix and committing should do the trick. But I just double-checked that the lint-staged setup should be automatically reformatting when committing. Is it possible your git commit actions has a --no-verify happening? Whether from your cli or some tool?