Bump rubocop-shopify from 2.8.0 to 2.9.0

Bumps [rubocop-shopify](https://github.com/Shopify/ruby-style-guide) from 2.8.0 to 2.9.0.
- [Release notes](https://github.com/Shopify/ruby-style-guide/releases)
- [Commits](Shopify/ruby-style-guide@v2.8.0...v2.9.0)

---
updated-dependencies:
- dependency-name: rubocop-shopify
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>