Enable ci_config_merged_tab feature by default

This enabled the ci_config_merged_tab feature by default
and adds the changelog.

changelogs/unreleased/feature-flag-enable-ci-config-merged-tab.yml

+---
+title: Add merged Yaml tab feature to Pipeline Editor
+merge_request: 54223
+author:
+type: added

config/feature_flags/development/ci_config_merged_tab.yml

@@ -5,4 +5,4 @@ rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/301103
 milestone: '13.9'
 type: development
 group: group::pipeline authoring
-default_enabled: false
+default_enabled: true