Remove allows_nil: false setting in default_value_for

The default_value_for gem uses the `<column>_changed?` method
to determine if the default value should be applied. For new records,
the `<column>_changed?` method returns false if the value of the
column is the same as the database default.

One workaround is to remove the `allows_nil: false` in the
`default_value_for` setting for container_registry_access_level so
that default_value_for does not overwrite the database default value.

This bug should be fixed by
https://github.com/FooBarWidget/default_value_for/pull/90.

The container_registry_access_level column has a null: false
constraint, so removing the allows_nil: false from the
default_value_for should not be a problem.

app/models/project_feature.rb

@@ -79,7 +79,7 @@ class ProjectFeature < ApplicationRecord
     end
   end
 
-  default_value_for(:container_registry_access_level, allows_nil: false) do |feature|
+  default_value_for(:container_registry_access_level) do |feature|
     if gitlab_config_features.container_registry
       ENABLED
     else

spec/factories/projects.rb

@@ -67,21 +67,11 @@ FactoryBot.define do
         pages_access_level: evaluator.pages_access_level,
         metrics_dashboard_access_level: evaluator.metrics_dashboard_access_level,
         operations_access_level: evaluator.operations_access_level,
-        analytics_access_level: evaluator.analytics_access_level
+        analytics_access_level: evaluator.analytics_access_level,
+        container_registry_access_level: evaluator.container_registry_access_level
       }
 
       project.build_project_feature(hash)
-
-      # This is not included in the `hash` above because the default_value_for in
-      # the ProjectFeature model overrides the value set by `build_project_feature` when
-      # evaluator.container_registry_access_level == ProjectFeature::DISABLED.
-      #
-      # This is because the default_value_for gem uses the <column>_changed? method
-      # to determine if the default value should be applied. For new records,
-      # <column>_changed? returns false if the value of the column is the same as
-      # the database default.
-      # See https://github.com/FooBarWidget/default_value_for/blob/release-3.4.0/lib/default_value_for.rb#L158.
-      project.project_feature.container_registry_access_level = evaluator.container_registry_access_level
     end
 
     after(:create) do |project, evaluator|