Support PHP8 and update GitHub Actions.

[Yozhef]

No description provided.

[Yozhef]

@Jean85 could you check?

[Jean85]

If the issue is the same as in getsentry/sentry-php#1150, PHPStan has troubles running under PHP 8 due to imprecise function signatures. Try to run it under 7.4 for now, I'll run a separate PR to investigate that (and eventually report bugs upstream).

As for the CI that is failing, you're tripping over 2 additional errors that are caught by Sentry, and both are reported in the CI.

[Yozhef]

@Jean85
I ran PHPStan on 7.4.13 but it, unfortunately, didn't help, but in the local machine all without errors.

What accrued additional problems in the CI - were added Deprecated Message.
Tests (7.2, 3.4.*, Symfony 3.4, disabled) && Tests (8.0, --prefer-lowest, false, false, false)

Wrong number of events sent:
b6431231fb48408e84b2d0f110c9eac2: User Notice: This is an intentional notice
###
70a61080c25c4704abc680192ea2a81d: Deprecated: Function libxml_disable_entity_loader () is deprecated
###
4b6ba3285c0b456bb377a11023eaf218: Deprecated: Function libxml_disable_entity_loader () is deprecated
###

[Jean85]

I've rerun CI on the branch and it's still green: https://github.com/getsentry/sentry-symfony/actions/runs/391772879

Could you please split this PR in two? Reorganization of the actions following Sentry-php repo example is good, but it should be done separately so we can find the root cause of this failures.

[Yozhef]

@Jean85 yes, thanks.

[Jean85]

Closing since #399 got the job done.