Compatybility with PHP 5 broken

[alecpl]

https://github.com/roundcube/roundcubemail/actions/runs/9146253289/job/25146201300

It looks that the version 0.3.4 does not work on PHP5, the composer.json has no version constraint. Also Roundcube 1.5 LTS should work on PHP5 too.

I suppose for now I'll use 0.3.3 for the Roundcube 1.5.7 release I'm working on, but maybe we should fix the issue and release 0.3.4.1.

[mvorisek]

For Roundcubemail branches supporting PHP5 (in other words incompatible with the current plugin-installer), you can add composer dependency conflict into the Roundcubemail composer.json.