Disable keyset pagination for branches by default

It seems that with this feature flag enabled, pagination doesn't work
correctly in conjunction with a search. The FF is already disabled on
GitLab.com, but disabling it in the YAML file means that self-managed
instances will also be protected from the security issue (unless they
explicitly opt-in to some beta code, of course).

Changelog: security

app/controllers/projects/branches_controller.rb

@@ -185,7 +185,7 @@ class Projects::BranchesController < Projects::ApplicationController
     # Here we get one more branch to indicate if there are more data we're not showing
     limit = @overview_max_branches + 1
 
-    if Feature.enabled?(:branch_list_keyset_pagination, project, default_enabled: true)
+    if Feature.enabled?(:branch_list_keyset_pagination, project, default_enabled: :yaml)
       @active_branches =
         BranchesFinder.new(@repository, { per_page: limit, sort: sort_value_recently_updated })
           .execute(gitaly_pagination: true).select(&:active?)

app/services/projects/branches_by_mode_service.rb

@@ -37,7 +37,7 @@ class Projects::BranchesByModeService
   def use_gitaly_pagination?
     return false if params[:page].present? || params[:search].present?
 
-    Feature.enabled?(:branch_list_keyset_pagination, project, default_enabled: true)
+    Feature.enabled?(:branch_list_keyset_pagination, project, default_enabled: :yaml)
   end
 
   def fetch_branches_via_offset_pagination

changelogs/unreleased/security-322500-disable-gitaly-branch-pagination-ff-by-default.yml

+---
+title: Disable keyset pagination for branches by default
+merge_request:
+author:
+type: security

config/feature_flags/development/branch_list_keyset_pagination.yml

@@ -5,4 +5,4 @@ rollout_issue_url:
 milestone: '13.2'
 type: development
 group: group::source code
-default_enabled: true
+default_enabled: false

lib/gitlab/pagination/gitaly_keyset_pager.rb

@@ -26,11 +26,11 @@ module Gitlab
       private
 
       def keyset_pagination_enabled?
-        Feature.enabled?(:branch_list_keyset_pagination, project, default_enabled: true) && params[:pagination] == 'keyset'
+        Feature.enabled?(:branch_list_keyset_pagination, project, default_enabled: :yaml) && params[:pagination] == 'keyset'
       end
 
       def paginate_first_page?
-        Feature.enabled?(:branch_list_keyset_pagination, project, default_enabled: true) && (params[:page].blank? || params[:page].to_i == 1)
+        Feature.enabled?(:branch_list_keyset_pagination, project, default_enabled: :yaml) && (params[:page].blank? || params[:page].to_i == 1)
       end
 
       def paginate_via_gitaly(finder)