Allow symfony/cache-contracts 3

[garak]

Currently, only versions 1 and 2 are allowed.

[ste93cry]

The newer version added mixed as return type to different methods and I'm not sure how much easy would be for us to support both it and the older versions. A PR is more than welcome though

[Jean85]

We may get away with it only with a conditional declaration of the class. It's impossible to support 1 and 3 with the same piece of code...

Also, : mixed is supported since PHP 8, another reason to declare that conditionally.

[ste93cry]

Fixed in #588. @garak would you mind testing it out and see if everything works as expected?

[garak]

Works for me, thank you!

[Jean85]

Closing as fixed in 4.2.6