PHP 8.1 support

See ongoing work in the 4.x branch

https://www.drupal.org/project/provision/releases/4.1.x-dev →

I haven't tested 8.1 yet, but 8.0 works for me.

For nginx, I had to do a small patch on provision, which I committed to 7.x-3.x.

See https://www.drupal.org/project/hostmaster/issues/3317637#comment-14848861 → - for instructions on how to update Aegir from source.