Bump postcss-custom-properties from 8.0.6 to 8.0.8
Bump postcss-custom-properties from 8.0.6 to 8.0.8

Bumps [postcss-custom-properties](https://github.com/postcss/postcss-custom-properties) from 8.0.6 to 8.0.8.
- [Release notes](https://github.com/postcss/postcss-custom-properties/releases)
- [Changelog](https://github.com/postcss/postcss-custom-properties/blob/master/CHANGELOG.md)
- [Commits](https://github.com/postcss/postcss-custom-properties/commits)

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@dependabot.com>
jeffjewiss/ember-cli-postcss,jeffjewiss/ember-cli-postcss