Simplify pending builds table by removing updated_at

This applies another patch from @abrandl that only adds `created_at`
instead of full timestamps.

db/migrate/20210512120122_add_pending_builds_table.rb

@@ -3,9 +3,9 @@
 class AddPendingBuildsTable < ActiveRecord::Migration[6.0]
   def up
     create_table :ci_pending_builds, id: false do |t|
-      t.timestamps_with_timezone null: false
       t.references :build, primary_key: true, default: nil, index: false, foreign_key: { to_table: :ci_builds, on_delete: :cascade }
       t.references :project, index: true, null: false, foreign_key: { on_delete: :cascade }
+      t.datetime_with_timezone :created_at, null: false, default: -> { 'NOW()' }
     end
   end
 

db/structure.sql

@@ -10753,10 +10753,9 @@ CREATE SEQUENCE ci_namespace_monthly_usages_id_seq
 ALTER SEQUENCE ci_namespace_monthly_usages_id_seq OWNED BY ci_namespace_monthly_usages.id;
 
 CREATE TABLE ci_pending_builds (
-    created_at timestamp with time zone NOT NULL,
-    updated_at timestamp with time zone NOT NULL,
     build_id bigint NOT NULL,
-    project_id bigint NOT NULL
+    project_id bigint NOT NULL,
+    created_at timestamp with time zone DEFAULT now() NOT NULL
 );
 
 CREATE TABLE ci_pipeline_artifacts (