PHP version numbers need to be strings: If the version `8.0` were
treated as a number, the YAML parser would happily convert it to `8`,
hence dropping the minor version number.
Having a consistent structure and order between the CI jobs and the
code coverage generation job makes understanding the GitHub Actions
configuration easier.
It also allows having a shared Composer cache between the non-coverage
jobs and the coverage job.
There is no need to have the code coverage data directly generated by
the tests run in a non-hidden directory. So now the code coverage is
recorded within `.Build/coverage`.
Only the merged coverage data still needs to be located in `build/logs`
as Coveralls (which we're planning to use) relies on this.
The installed Composer packages will be different depending on the
major TYPO3 version being tested and on the strategy for the
dependencies (highest, lowest). So it makes sense to have
separate Composer caches for these versions in order to avoid
cache misses.
Composer versions 2.2.0 and 2.2.1 have a bug that breaks PHPUnit.
Until version 2.2.2 is released with a fix, we need to keep to
Composer 2.1 to keep the tests from breaking.
See https://github.com/composer/composer/issues/10387 for details.
