Define minimum target PHP version for PHPStan?

[JakeQZ]

From #1212 (comment)

• I think the new PHPStan error arises because it would now be possible to have CSSListItem|array as a native parameter type - but not in PHP 7.2 -
  • It would be nice if PHPStan would allow specification of a target PHP version separately from the version it runs on, so it would not generate errors that can't be fixed for the minimum target PHP version;
    • IIRC, this was not possible with Psalm, but maybe it is possible with PHPStan;
    • Running PHPStan on PHP 7.2 would almost certainly not permit the latest version.

Need to first investigate if this is possible. If it is, then decide if we also want to receive the warnings applicable to later PHP versions, somewow. If it's not possible, then there's nothing we can do.

[oliverklee]

https://phpstan.org/config-reference#phpversion