Bump nth-check, cssnano and gulp-imagemin

Bumps [nth-check](https://github.com/fb55/nth-check) to 2.1.1 and updates ancestor dependencies [nth-check](https://github.com/fb55/nth-check), [cssnano](https://github.com/cssnano/cssnano) and [gulp-imagemin](https://github.com/sindresorhus/gulp-imagemin). These dependencies need 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 `cssnano` from 4.1.11 to 5.1.13
- [Release notes](https://github.com/cssnano/cssnano/releases)
- [Commits](https://github.com/cssnano/cssnano/compare/v4.1.11...cssnano@5.1.13)

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: cssnano
  dependency-type: direct:development
- dependency-name: gulp-imagemin
  dependency-type: direct:development
...

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Author: dependabot[bot]