Enable fill_in_mr_template by default

It's been a couple of days since the feature flag has been
globally enabled on production and no issues were found.

This enables the feature flag by default and will be removed
when the feature is ready to be released as GA.

Changelog: changed

config/feature_flags/development/fill_in_mr_template.yml

@@ -5,4 +5,4 @@ rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/412796
 milestone: '16.1'
 type: development
 group: group::code review
-default_enabled: false
+default_enabled: true