Remove unnecessary release prep gems

Prior to this commit, we added the concurrent-ruby gem in c2e7fa2 to
compensate for issues relating to the Docker image used for release
preparation.

Since then, we have switched to piggy-backing off of the CAT team's
release prep GitHub Action (see 65a59a3). That action stopped using the
problematic Docker image in mid-2023
(puppetlabs/cat-github-actions@4a83f93), so we no longer need to manage
concurrent-ruby.

This commit also removes the github_changelog_generator gem, as it has been
superceded by the gh-changelog tool to generate changelogs for releases.

Author: mhashizume

.sync.yml

@@ -39,10 +39,6 @@ Gemfile:
       - gem: beaker-puppet
         from_env: BEAKER_PUPPET_VERSION
         version: '~> 1.22'
-      - gem: github_changelog_generator
-        version: '= 1.16.4'
-      - gem: concurrent-ruby
-        version: '= 1.1.10'
       - gem: async
         version: '~> 1'
       - gem: beaker-module_install_helper

Gemfile

@@ -40,8 +40,6 @@ group :development do
   gem "beaker-hostgenerator"
   gem "beaker-rspec"
   gem "beaker-puppet", *location_for(ENV['BEAKER_PUPPET_VERSION'] || '~> 1.22')
-  gem "github_changelog_generator", '= 1.16.4',                                 require: false
-  gem "concurrent-ruby", '= 1.1.10',                                            require: false
   gem "async", '~> 1',                                                          require: false
   gem "beaker-module_install_helper",                                           require: false
   gem "beaker-puppet_install_helper",                                           require: false