(maint) Pins github_changelog_generator gem

This commit pins the github_changelog_generator gem to 1.15.2, as
there are issues with newer versions.

Author: mhashizume

.sync.yml

@@ -21,6 +21,7 @@ Gemfile:
         from_env: BEAKER_PUPPET_VERSION
         version: '~> 1.22'
       - gem: github_changelog_generator
+        version: '=1.15.2'
       # We can unpin async when we move to Ruby 3
       - gem: async
         version: '~> 1'