Bump the postcss group with 2 updates

Bumps the postcss group with 2 updates: [cssnano](https://github.com/cssnano/cssnano) and [cssnano-preset-default](https://github.com/cssnano/cssnano).


Updates `cssnano` from 7.0.3 to 7.0.4
- [Release notes](https://github.com/cssnano/cssnano/releases)
- [Commits](https://github.com/cssnano/cssnano/compare/cssnano@7.0.3...cssnano@7.0.4)

Updates `cssnano-preset-default` from 7.0.3 to 7.0.4
- [Release notes](https://github.com/cssnano/cssnano/releases)
- [Commits](https://github.com/cssnano/cssnano/compare/cssnano-preset-default@7.0.3...cssnano-preset-default@7.0.4)

---
updated-dependencies:
- dependency-name: cssnano
  dependency-type: direct:development
  update-type: version-update:semver-patch
  dependency-group: postcss
- dependency-name: cssnano-preset-default
  dependency-type: direct:development
  update-type: version-update:semver-patch
  dependency-group: postcss
...

Signed-off-by: dependabot[bot] <support@github.com>

package.json

@@ -72,7 +72,7 @@
     "@rollup/plugin-replace": "^5.0.7",
     "@rollup/plugin-terser": "^0.4.4",
     "autoprefixer": "^10.4.19",
-    "cssnano": "^7.0.3",
+    "cssnano": "^7.0.4",
     "cssnano-preset-default": "^7.0.1",
     "govuk-prototype-kit": "^13.16.2",
     "gulp": "^5.0.0",