Update cookstyle requirement from ~> 8.6 to ~> 9.0

Updates the requirements on [cookstyle](https://github.com/chef/cookstyle) to permit the latest version.
- [Release notes](https://github.com/chef/cookstyle/releases)
- [Changelog](https://github.com/chef/cookstyle/blob/main/CHANGELOG.md)
- [Commits](chef/cookstyle@v8.6.0...v9.0.0)

---
updated-dependencies:
- dependency-name: cookstyle
  dependency-version: 9.0.0
  dependency-type: direct:development
...

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

Author: dependabot[bot]

Gemfile

@@ -18,7 +18,7 @@ group :development, :test do
 end
 
 group :style do
-  gem "cookstyle", "~> 8.6"
+  gem "cookstyle", "~> 9.0"
 end
 
 if ENV["GEMFILE_MOD"]