fix deprecation for carbon 3

[tgabi333]

my laravel config is setup to log deprecations, i got this:

production.WARNING: ErrorException: Implicit conversion from float -1.9990081062193847 to int loses precision 
in /app/vendor/lucasdotvin/laravel-soulbscription/src/Enums/PeriodicityType.php:24

As Carbon documentation says:

diffAsCarbonInterval() and diffIn*() (and floatDiffIn*() for versions < 3 when diffIn*() methods returned integer values,
since Carbon 3, they are deprecated as diffIn*() already return floating number, and integer values from it can easily be
obtained with an explicit cast (int)).

[tgabi333]

ping @lucasdotvin

[tgabi333]

@lucasdotvin please