Disable parallel matrix feature flag

https://gitlab.com/gitlab-org/gitlab/-/merge_requests/33705 introduced a
production incident
(https://gitlab.com/gitlab-com/gl-infra/production/-/issues/2442). Disable
this feature flag by default until we validate this feature is stable.

lib/gitlab/ci/features.rb

@@ -67,7 +67,7 @@ module Gitlab
       end
 
       def self.parallel_matrix_enabled?
-        ::Feature.enabled?(:ci_parallel_matrix_enabled, default_enabled: true)
+        ::Feature.enabled?(:ci_parallel_matrix_enabled)
       end
 
       def self.bulk_insert_on_create?(project)