Add php-74 to static tests

[sreichel]

Description (*)

composer.json allows PHP7.0-7.4, so we should also cover 7.4 with static tests.

• should be merged after Updated lib Net/IDNA2 to latest version #1181, Mage_Eav - Fix PHP 7.4 deprecation: array/string curly braces access #1182
• made 7.4 non-experimental
• added 7.0-7.3 experimental tests, with included lib-directories (PEAR maybe will be removed, not sure about phpseclib, but lib/Zend should be covered)

Contribution checklist (*)

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All automated tests passed successfully (all builds are green)

[Flyingmana]

do we really need the checks for all versions of php7?
Maybe only the oldest and newest? Because, each of them takes 5min, and I dont think its likely to have a failure in between them

[kkrieger85]

If we'd remove checks, than I'd ensure the latest supported versions are running. See https://www.php.net/supported-versions.php