BookStack - upcoming release will require PHP8.0+

[JedMeister]

As posted by @ssddanbrown over on https://github.com/turnkeylinux-apps/bookstack/issues/1 (I forgot to disable issues over there - Doh!).

Hello 👋
I'm Dan, the creator/lead-dev of BookStack.

Just wanted to warn about an upcoming change to the minimum supported PHP version for BookStack.
It'll be changing from php7.4 to php8.0 in the next feature release (Likely due end of the month).
Thought I'd raise that here as I'm not exactly sure how PHP versions are managed for turnkey, and I couldn't immediately see a specific PHP version in use from the sources here.

As I noted in response, we'll aim to rebuild BookStack ASAP with newer PHP in preparation.