Enable the FF ci_modified_paths_of_external_prs

It's already enabled on GitLab.com

Changelog: fixed

config/feature_flags/development/ci_modified_paths_of_external_prs.yml

@@ -5,4 +5,4 @@ rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/330605
 milestone: '13.12'
 type: development
 group: group::pipeline authoring
-default_enabled: false
+default_enabled: true