[8.x] Ensure int type of getSeconds() output

[kylekatarnls]

Hello 👋 In Carbon 3, diff* will return a float (current behavior of floatDiff*) so value is never truncated and result will be less surprising when having 0.999 day or the like.

getSeconds() PHPDoc says int and it sounds like the intent behind the (int) cast was to ensure it, but due to operator precedence, it's actually doing ((int) $duration) > 0 ? $duration : 0 so the type of the return is actually not in control of the method. Make the cast as the last step ensure it.

Side note: it's present from 6.x to master, I can change the branch if 6.x/7.x should still be supported.

Thanks 🙏

[GrahamCampbell]

6.x is just security fixes now, and 7.x it completely EOL. 8.x is the right place for this.