Check php compatibility of app store app releases

Apps might increase the minimum php version requirement, in which case
an update could break the app or even the whole instance. We must not
install those releases, or better, don't even show them for
update/installation. This extends the app fetcher code to filter out the
releases that are not installable.

The filter respects minimum and maximum requirements. E.g. apps that are
still only released for php7.3 won't show up for php7.4 instances. This
behavior is new but if an app lists an explicit version requirement,
then we ought to repect that.

Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>