Update github_changelog_generator requirement from ~> 1.16.4 to ~> 1.18.0

Updates the requirements on [github_changelog_generator](https://github.com/github-changelog-generator/github-changelog-generator) to permit the latest version.
- [Release notes](https://github.com/github-changelog-generator/github-changelog-generator/releases)
- [Changelog](https://github.com/github-changelog-generator/github-changelog-generator/blob/master/CHANGELOG.md)
- [Commits](github-changelog-generator/github-changelog-generator@v1.16.4...v1.18.0)

---
updated-dependencies:
- dependency-name: github_changelog_generator
  dependency-version: 1.18.0
  dependency-type: direct:development
...

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

Author: dependabot[bot]

Gemfile

@@ -29,5 +29,5 @@ end
 
 group(:release, optional: true) do
   gem 'faraday-retry', '~> 2.1', require: false
-  gem 'github_changelog_generator', '~> 1.16.4', require: false
+  gem 'github_changelog_generator', '~> 1.18.0', require: false
 end