Bump postcss-discard-comments from 6.0.2 to 7.0.4

Bumps [postcss-discard-comments](https://github.com/cssnano/cssnano) from 6.0.2 to 7.0.4.
- [Release notes](https://github.com/cssnano/cssnano/releases)
- [Commits](https://github.com/cssnano/cssnano/compare/postcss-discard-comments@6.0.2...postcss-discard-comments@7.0.4)

---
updated-dependencies:
- dependency-name: postcss-discard-comments
  dependency-version: 7.0.4
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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Author: dependabot[bot]