Enable merged branch names flag by default

app/models/repository.rb

@@ -919,7 +919,7 @@ class Repository
     # This is only used in a few places, notably app/services/branches/delete_merged_service.rb,
     # and it could potentially result in a very large cache/performance issues with the current
     # implementation.
-    skip_cache = branch_names.empty? || Feature.disabled?(:merged_branch_names_redis_caching)
+    skip_cache = branch_names.empty? || Feature.disabled?(:merged_branch_names_redis_caching, default_enabled: true)
     return raw_repository.merged_branch_names(branch_names) if skip_cache
 
     cached_branch_names = cache.read(:merged_branch_names)