WordPress registration message

This message is not part of the WordPress core. I confirmed this suspicion by searching for the message within the WordPress core files. When it was not found, I then searched the message using a search engine. It appears that you are using the New User Approve Message plugin.

This message is defined in new-user-approve/includes/messages.php#L33

/**
 * The default message that will be shown to the user after registration has completed.
 *
 * @return string
 */
function nua_default_registration_complete_message() {
    $message = sprintf( __( 'An email has been sent to the site administrator. The administrator will review the information that has been submitted and either approve or deny your request.', 'new-user-approve' ) );
    $message .= ' ';
    $message .= sprintf( __( 'You will receive an email with instructions on what you will need to do next. Thanks for your patience.', 'new-user-approve' ) );
    $message = apply_filters( 'new_user_approve_pending_message_default', $message );
    return $message;
}

You can modify this message using the filter new_user_approve_pending_message_default.

add_filter( 'new_user_approve_pending_message_default',
            'wpse_new_user_approve_pending_message_default' );
function wpse_new_user_approve_pending_message_default( $message ) {
    // Customize $new_message as needed and return it.
    $new_message = sprintf( __( 'New message...', 'text-domain' ) );

    return $new_message;
}