Add 'woocommerce_order_status_on-hold_to_processing', to $email_actions

This addition would stop users from editing the core, as there is currently no eloquent way to edit the $email_actions array. Especially useful for creating email notifications for confirming payment (BACS).
Merge pull request #6543 from scottdurban/patch-1

Add 'woocommerce_order_status_on-hold_to_processing', to $email_actions