Remove `create_function` or check for existence of that function

[roborourke]

In PHP 8.0 create_function has been removed and it has been deprecated since PHP 7.2.

We should update the logic for the vary cache functions, either by removing them as an option, removing any security checks and allowing anonymous functions or by switching Altis to use Robin and abandoning this fork.

[mikelittle]

Here's an upstream solution d4f617b but we may need to check security settings