ci: make CI pass

[simPod]

• rework phpstan
• check multiple php versions
• fix errors

Signed-off-by: Simon Podlipsky simon@podlipsky.net

https://github.com/PromPHP/prometheus_client_php/pull/99 broke CI, this aims to fix it since all other PRs now fail which is irrelevant to them.

[simPod]

Also we should unmark

P7.2 - prefer-stable - ubuntu-latest P7.3 - prefer-stable - ubuntu-latest

as required checks

[LKaemmerling]

Thank you!