Remove RepublishingEvent bulk default value

Now that we've updated existing `RepublishingEvent`s with no `bulk`
value to set the value to `false`, we'd rather not provide the default
option and ensure the value is set explicitly where `RepublishingEvent`s
are created.

db/migrate/20240529103123_make_republishing_events_bulk_nil_by_default.rb

@@ -0,0 +1,3 @@
+class MakeRepublishingEventsBulkNilByDefault < ActiveRecord::Migration[7.1]
+  change_column_default(:republishing_events, :bulk, nil)
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[7.1].define(version: 2024_05_29_093425) do
+ActiveRecord::Schema[7.1].define(version: 2024_05_29_103123) do
   create_table "assets", charset: "utf8mb3", force: :cascade do |t|
     t.string "asset_manager_id", null: false
     t.string "variant", null: false
@@ -865,7 +865,7 @@
     t.datetime "created_at", null: false
     t.datetime "updated_at", null: false
     t.string "content_id"
-    t.boolean "bulk", default: false, null: false
+    t.boolean "bulk", null: false
     t.integer "bulk_content_type"
     t.index ["user_id"], name: "index_republishing_events_on_user_id"
   end