Improve updating with autoinstall.conf.php

It'd be nice to make updating easier and less error prone for users. Currently it's hard to find much information regarding the use of autoinstall.conf.php/autoinstall.ini for updating. I want to change this.

1. Update https://git.ispconfig.org/ispconfig/ispconfig3/-/blob/develop/docs/autoinstall_samples/autoinstall.conf_sample.php -> add missing config variable: $autoupdate['mysql_master_port'] = '3306';
2. Pass --autoinstall option from ispconfig_update.sh and ispc update to update.php (see also https://forum.howtoforge.com/threads/ispconfig-update-script.87292/)
3. Write clear documentation on how to set this up for any ISPConfig server. Probably even with a script to help you set it up.