Horde_Date defense against float values in PHP 8.3+

[ralflang]

In PHP 8.3 and later some of the builtins we use for Horde_Date are more strict in which formats they accept than before.
There are some cases Horde_Date itself or a caller to Horde_Date passes floats where ints are expected.
As we cannot / should not modify the interfaces of lib/ in beta phase we will need to internally cast such values or throw exceptions so the caller gets fixed.

This needs to be covered with some tests BEFORE attempting to fix. We need to be very clear what are the desirable calculation outcomes and why we mean our methods to work THIS way. I will take this over.