Check raising requirements before 5.0 release

[mstilkerich]

Check if it makes sense to raise RCMCardDAV requirements particularly concerning PHP (and consequently supported roundcube versions) before RCMCardDAV 5.0. We already cannot run unittests for PHP versions older than 7.4 anymore because our dev dependencies require PHP 7.4 or higher.

[mstilkerich]

From packages statistics for 4.x installs, PHP 7.4 is still the dominant version (52%), so it appears to early to move directly to PHP 8.0. PHP 7.3 is only around 5%, however, and 7.1 and 7.2 don't appear at all.

PHP 7.4 is also the oldest version we are still able to use in static analysis with Psalm 5, and it has some nice new language features compared to 7.1, which makes it the best candidate to set as new minimum requirement.

Concerning roundcube, version 1.4 can still be used with PHP 7.4, later roundcube versions also support PHP 8. So we can keep the roundcube supported versions at starting from 1.4.

For the databases, I don't see any reason to raise the minimum requirements.