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!

https://github.com/tedious/Stash/blob/a2c9df16c77b13e1f90462f30597a772166eea8f/src/Stash/Session.php#L229

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

[tedivm]

If you make a PR I'll merge it.