[5.9] Drop Carbon v1

[driesvints]

Carbon v1 is now warning to drop support and upgrade to v2 instead so it's best that we drop support for it in newer versions. See briannesbitt/Carbon#1685

[kylekatarnls]

Thanks @driesvints it will help me to focus maintenance efforts on Carbon 2.

@bobdenotter It's for 5.9, not 5.8. But indeed, the minimum PHP version becomes 7.1.8 de facto and should be reflected in the composer.json and doc.

For the record, we need 7.1.8 because of this bug fixed in this version:

Fixed bug #74852 (property_exists returns true on unknown DateInterval property).
https://bugs.php.net/bug.php?id=74852

[kylekatarnls]

I will check if we can work around this bug and support 7.1.3.

[driesvints]

@bobdenotter @kylekatarnls the next Laravel version has a minimum requirement of PHP 7.2 so we should be good.