Use PHP 8.0 in CI due to failing runs

[dennisameling]

I keep getting these daily emails that CI is failing for this repo. Here's a failing run: https://github.com/mautic/api-library/actions/runs/3922881304

Things are failing because PHP 8.0 is now required. This PR updates PHP to that version, which hopefully should fix things.

[dennisameling]

Ok so the good news is that CI runs again, but there's a few failing tests (one of them being a 500 Server Error from Mautic itself). Does anyone have the bandwidth to look into those? 😊

[escopecz]

Thank you Dennis! I'll try to find some time to look into the failing tests this week and merge this. We plan to release new major version of this library so this is a good start.

[escopecz]

I fixed the failing tests in this repo and one more was failing due to PHP 8 strictness on Mautic's side. Here is the fix for that: mautic/mautic#11873

[codecov-commenter]

Codecov Report

Merging #289 (979db08) into main (494807f) will increase coverage by 1.63%.
The diff coverage is n/a.

❗ Current head 979db08 differs from pull request most recent head 70b6afd. Consider uploading reports for the commit 70b6afd to get more accurate results

@@             Coverage Diff              @@
##               main     #289      +/-   ##
============================================
+ Coverage     49.94%   51.58%   +1.63%     
  Complexity      408      408              
============================================
  Files            30       30              
  Lines           973     1103     +130     
============================================
+ Hits            486      569      +83     
- Misses          487      534      +47     

Impacted Files	Coverage Δ
lib/Api/Forms.php	13.33% <0.00%> (-20.00%)	⬇️
lib/Api/CampaignEvents.php	58.33% <0.00%> (-11.67%)	⬇️
lib/QueryBuilder/QueryBuilder.php	50.00% <0.00%> (-4.06%)	⬇️
lib/Auth/OAuth.php	0.00% <0.00%> (ø)
lib/Response.php	92.72% <0.00%> (+0.27%)	⬆️
lib/QueryBuilder/WhereBuilder.php	90.58% <0.00%> (+0.58%)	⬆️
lib/Api/Api.php	79.35% <0.00%> (+1.12%)	⬆️
lib/Auth/AbstractAuth.php	74.73% <0.00%> (+1.40%)	⬆️
lib/Api/Contacts.php	94.04% <0.00%> (+3.66%)	⬆️
lib/Api/Stats.php	54.54% <0.00%> (+17.04%)	⬆️

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[dennisameling]

Thanks @escopecz!! Let's merge this 🔥