DEV: Promote uppy backup uploader to primary uploader (#15363)

This commit removes the enable_experimental_backup_uploader site
setting and the flags in backups-index.hbs to make the uppy
backup uploader the main one from now on.

A follow-up commit will delete the old backup uploader code and
also remove resumable.js from the project.

db/migrate/20211220023034_remove_experimental_backup_uploader_setting.rb

@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+class RemoveExperimentalBackupUploaderSetting < ActiveRecord::Migration[6.1]
+  def up
+    execute <<~SQL
+      DELETE FROM site_settings
+      WHERE name = 'enable_experimental_backup_uploader'
+    SQL
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end