Enable async_merge_request_check_mergeability by default

The feature has been enable on GitLab.com for a few days now
and it's been working without issues.

To include in the final release, should be enabled by default.
This way, self-managed installs can still disable it if they
encounter some issues we haven't seen.

app/models/merge_request.rb

@@ -866,7 +866,7 @@ class MergeRequest < ApplicationRecord
 
     check_service = MergeRequests::MergeabilityCheckService.new(self)
 
-    if async && Feature.enabled?(:async_merge_request_check_mergeability, project)
+    if async && Feature.enabled?(:async_merge_request_check_mergeability, project, default_enabled: true)
       check_service.async_execute
     else
       check_service.execute(retry_lease: false)

changelogs/unreleased/199912-ff-enable-by-default.yml

+---
+title: Enable async_merge_request_check_mergeability by default
+merge_request: 31196
+author:
+type: other