Remove unused gulp-css-url-adjuster dependency

[whabanks]

Summary | Résumé

This PR is part 2 of 2 (part 1) to resolve a critical security vulnerability with versions of lodash that are < 4.5.0.

Why?

The dependency tree:

gulp-css-url-adjuster
|__ gulp-util
|____ lodash (< v4.5.0)

gulp-css-url-adjuster is dependent on gulp-util which has been deprecated. The authors recommend moving away from using it. Additionally the authors ask that if your project has a dependency which uses gulp-util to create an issue in the dep's repo recommending it's removal and offering replacements for certain API calls.

Initially this was my plan, however, there has not been a commit in the repo for over 8 years and there are issues that have been open for 6+ years. I expect that if we opened an issue it would receive the same treatment as maintenance of the dep seems to have halted.

Lastly, while we require the dependency in our gulp.js we do not appear to call or utilize it, and removing it has had no apparent affect on the building of our dev containers or within the running application.

An aside

It may be beneficial for relations to open a PR in GDS' Admin repo, or otherwise share this with them as they appear to be in the same boat.

Testing

• Build the dev container, there should be no errors.
• Run the app and poke around the site for anything CSS related that seems out of place or incorrect.

[github-actions]

🧪 Review environment

https://v2e3mqw72jgre2wtcbuyrkbug40fmwuk.lambda-url.ca-central-1.on.aws/

[andrewleith]

Nice! Good idea on letting GDS know, we have a shared slack channel with them as well. Could ask @jimleroyer his thoughts on the best way to share this.

As for this change, I compared the contents of the generated assets in this branch (all the files in app/static) with those in the main branch and they are identical, so I believe you are correct in that we are not using this anywhere and removing it is innocuous! 🎉