v2.8.18-canary.20

<!-- Release notes generated using configuration in .github/release.yml at v2.8.18-canary.20 -->

What's Changed

Changelog

• feat: Deprecate --parallel flag by @anthonyshew in https://github.com/vercel/turborepo/pull/12357

Full Changelog: https://github.com/vercel/turborepo/compare/v2.8.18-canary.19...v2.8.18-canary.20