Adds compatibility for Laravel 8

[vesper8]

This tiny change to the composer.json file adds compatibility to Laravel 8 (tested and works great).

The requirements do not mention it but because of the inclusion of "illuminate/support": "^9.0",, this currently only works for Laravel 9 or higher.

Please consider merging this PR so that I don't have to manage a fork just to make it work on Laravel 8.

It should be noted that Laravel Nova v4 explicitly supports both Laravel 8 & 9

Many thanks for your awesome calendar!

[wdelfuego]

Thanks, @vesper8 ! Will merge and release soon.

[heshamwatany]

when will it be released?

[wdelfuego]

Just merged and released in v1.6.4. Sorry for the delay and thanks for the reminder!