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.
The CI workflow with predefined GitHub actions had an empty `on:`
element, which is invalid. Use the correct event for manually
triggered workflow runs instead.
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.
