minor #5689 Calculate code coverage on PHP 8 (kubawerlos)

This PR was squashed before being merged into the 2.19 branch.

Discussion
----------

Calculate code coverage on PHP 8

It's almost a half a year since the release, we can use PHP 8 to calculate code coverage e.g. to avoid situation like in #5685 where it shows that coverage dropped.

Commits
-------

acc1806 Calculate code coverage on PHP 8

.github/workflows/ci.yml

@@ -35,12 +35,6 @@ jobs:
             PHP_CS_FIXER_TEST_USE_LEGACY_TOKENIZER: 1
             SYMFONY_DEPRECATIONS_HELPER: 'disabled'
 
-          - operating-system: 'ubuntu-20.04'
-            php-version: '7.4'
-            job-description: 'with calculating code coverage'
-            calculate-code-coverage: 'yes'
-            phpunit-flags: '--testsuite coverage --exclude-group covers-nothing --coverage-clover build/logs/clover.xml'
-
           - operating-system: 'ubuntu-20.04'
             php-version: '7.4'
             job-description: 'with deployment'
@@ -53,7 +47,9 @@ jobs:
 
           - operating-system: 'ubuntu-20.04'
             php-version: '8.0'
-            composer-flags: '--ignore-platform-req=php' # as this is a version not yet officially supported by PHP CS Fixer
+            job-description: 'with calculating code coverage'
+            calculate-code-coverage: 'yes'
+            phpunit-flags: '--testsuite coverage --exclude-group covers-nothing --coverage-clover build/logs/clover.xml'
 
           - operating-system: 'windows-latest'
             php-version: '7.4'