Bump nth-check, critical and gulp-imagemin

Bumps [nth-check](https://github.com/fb55/nth-check), [critical](https://github.com/addyosmani/critical) and [gulp-imagemin](https://github.com/sindresorhus/gulp-imagemin). These dependencies needed to be updated together.

Updates `nth-check` from 1.0.2 to 2.1.1
- [Release notes](https://github.com/fb55/nth-check/releases)
- [Commits](fb55/nth-check@v1.0.2...v2.1.1)

Updates `critical` from 2.0.6 to 5.0.2
- [Release notes](https://github.com/addyosmani/critical/releases)
- [Changelog](https://github.com/addyosmani/critical/blob/master/CHANGELOG.md)
- [Commits](addyosmani/critical@v2.0.6...v5.0.2)

Updates `gulp-imagemin` from 7.1.0 to 8.0.0
- [Release notes](https://github.com/sindresorhus/gulp-imagemin/releases)
- [Commits](sindresorhus/gulp-imagemin@v7.1.0...v8.0.0)

---
updated-dependencies:
- dependency-name: nth-check
  dependency-type: indirect
- dependency-name: critical
  dependency-type: direct:development
- dependency-name: gulp-imagemin
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <[email protected]>