Union type declaration in Session.php is available as of PHP 8.0.0

[mitkola]

Stash cache from version 0.17.0+ and later require PHP 7+, but there are union type declaration
which is available as of PHP 8.0.0.
This lead to syntax error in PHP 7+ environments!

Stash/src/Stash/Session.php

Line 229 in a2c9df1

public function gc($maxlifetime) : int|false

I suggest to use mixed return type and update PHP doc header.

[tedivm]

If you make a PR I'll merge it.