Update github_changelog_generator to ~> 1.18

Replace fork/outdated version pins with the released 1.18 gem which
includes the duplicate commit processing fix upstream.

Signed-off-by: nmburgan <13688219+nmburgan@users.noreply.github.com>

Author: nmburgan

Gemfile

@@ -74,6 +74,5 @@ 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', github: 'smortex/github-changelog-generator', branch: 'avoid-processing-a-single-commit-multiple-time', require: false
+  gem 'github_changelog_generator', '~> 1.18', require: false
 end