DEV: Move tag_group cleanup from post_migrate -> migrate (#16434)

There is no need to wait until after the deploy for this cleanup. In fact, running it later will mean there could be a window of a few minutes during which the site is broken.

The only requirement is that it runs after the broken `20220401130745_create_category_required_tag_groups` migration.

Followup to 39a6de3d73

db/post_migrate/20220407195246_remove_category_required_tag_groups_without_tag_groups.rb

@@ -1,19 +0,0 @@
-# frozen_string_literal: true
-
-class RemoveCategoryRequiredTagGroupsWithoutTagGroups < ActiveRecord::Migration[6.1]
-  def up
-    execute <<~SQL
-      DELETE FROM category_required_tag_groups
-      WHERE id IN (
-        SELECT crtg.id
-        FROM category_required_tag_groups crtg
-        LEFT OUTER JOIN tag_groups tg ON crtg.tag_group_id = tg.id
-        WHERE tg.id IS NULL
-      )
-    SQL
-  end
-
-  def down
-    raise ActiveRecord::IrreversibleMigration
-  end
-end