Drop PHP < 8.0

[kylekatarnls]

Until then, we planned to keep Carbon 3.0 compatible with PHP 7.4. This will finally be dropped too so minimum PHP level of Carbon 3.0 will be PHP 8.0. Reasons leading to this decision are the following:

• Release will be a bit later than expected (more likely end of year) and so PHP 7.4 will be very close to its end of support date.
• Symfony 6.0 will require PHP 8.0 and we'll follow the pace.
• Laravel 9.0 required PHP >= 7.4 until recently, that was one of the reason to keep 7.4 which is no longer relevant.
• ⬆️ Make CarbonPeriod extend DatePeriod #1752 PHP 8.0 will allow CarbonPeriod to extend DatePeriod which is a great enhancement for the consistency of Carbon main classes suite which are all sub-class of native PHP ones.
• It will allow to merge 3.x and 4.x branches which mean less maintenance burden.

As always dropping compatibility for the next major release does not mean you should worry if you're on PHP 7.4 or lower. Carbon 2.x will continue to be compatible with PHP >= 7.1.8 and minor fixes and language patches will continue to be released even after 3.0 lives as stable.