function do_robots() should use admin_url()

[Hrohh]

Dear all,

we have /wp-admin/ hardcoded in do_robots() function. How can i use filter admin_url('admin-ajax.php') ?

<?php
        $site_url = parse_url( site_url() );
        $path     = ( ! empty( $site_url['path'] ) ) ? $site_url['path'] : '';
        $output  .= "Disallow: $path/wp-admin/\n";
        $output  .= "Allow: $path/wp-admin/admin-ajax.php\n";

Thank you.

Trac ticket: https://core.trac.wordpress.org/ticket/63467

### Description

This PR introduces the use of the admin_url() function to dynamically retrieve the admin URL, replacing previously hardcoded values.

### Screenshots

Installation	Before	After
Single Site
Multisite