Set projects_post_creation_worker feature flag to default

Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/326665
Introduced in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/58119

Use Projects::PostCreationWorker to asynchronously setup
PrometheusService by default.

changelogs/unreleased/326665_enable_projects_post_creation_worker.yml

+---
+title: Create prometheus service asynchronously by default when creating a project
+merge_request: 59273
+author:
+type: changed

config/feature_flags/development/projects_post_creation_worker.yml

@@ -5,4 +5,4 @@ rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/326665
 milestone: '13.11'
 type: development
 group: group::source code
-default_enabled: false
+default_enabled: true