use stable PHP 8 releases in the CI

[xabbuh]

No description provided.

[GuilhemN]

I pushed some commits to your branch, I hope you don't mind.

I upgraded the code to phpunit 9 as it's the only version supporting php 8. The ignore requirements is back due to JMS\serializer.
There are some failures remaining, I'm not sure yet where do they come from, they may be related to some changes in php 8.

[GuilhemN]

Wow the battle was tough... But I won against phpunit and php 8 :D

This is ready for me, I can also do the merge in 3.x if you want.

[DenchikBY]

Hi, do you plan rework annotations in this PR to support php 8 format?

[xabbuh]

If you are talking about support for PHP 8 attributes, that's not in the scope of this PR. But if you are interested in giving it a try, please feel free to open a separate pull request.

[kevinpapst]

Can we help to push this PR and the upcoming release that will include it?

[GuilhemN]

Oops sorry I just realized this PR was also targetting 2.x and I could have completed it instead of pushing to #2280... I guess I wasn't well awaken when I did that :x

Anyway it's merged now so too late I suppose... I'm closing this PR in favor of #2280 :)