Allow PHP 8.1

[Fredthelead]

• Changed the PHP ~8.0.0 constraint to ~8.0
• Ran composer update with composer 2.3.9 and PHP 8.0.21
• Added PHP 8.1 to the github pipes
• Updated psalm config and suppressed a false-positive

Fixes #721

[dfreudenberger]

Anything that needs to be done in order to get this merged & released? Happy to help if something is missing.

[Ocramius]

Release requires this work: https://github.com/Ocramius/ProxyManager/milestone/56

[Fredthelead]

The problem with this PR is that the tests won't work with locked dependencies on both php 8.0 and 8.1. Not sure how to solve this without removing some tests.

[Ocramius]

Supporting 8.0 and 8.1 at the same time might indeed be challenging.

I can suggest skipping some tests on 8.0, so we focus on releasing a version that works on both PHP 8.0 and 8.1, then drop 8.0 support in a new minor, which in turn removes all conditionals that disable some tests.

[jakublabno]

Hi, any update on this?

[Ocramius]

#738 (comment)