Upgrade PHP from v8.0 to v8.1

[binarykitchen]

Because new Nextcloud vulnerabilities have been reported and look scary, we should consider upgrading Nextcloud from the current v26 to the latest v27.

But because v27 requires PHP v8.1 we should do this in two steps, with this one PHP upgrade first:
https://docs.nextcloud.com/server/latest/admin_manual/release_notes/upgrade_to_27.html

The following links confirm these reported vulnerabilities:

• https://www.cvedetails.com/vulnerability-list/vendor_id-15913/Nextcloud.html
• https://www.heise.de/en/news/Nextcloud-Attackers-can-bypass-two-factor-authentication-9766141.html

In the past, two PRs have been made for this but never got merged:

• Migrate from php8.0 to php8.1 #2309
• Upgrade to php8.1 and upgrade old nextcloud versions using docker #2319

These PRs are probably behind master, so let's create a new PR and pick the best from both PRs while carefully testing everything locally.

Suggestions welcome. Discussion is open here.