[5.9/6.0] Limit PHPUnit to version 8.0 and above.
[5.9/6.0] Limit PHPUnit to version 8.0 and above.

The changes has been done on `develop` branch for `laravel/laravel`. It is logical that we does this as well on `laravel/framework`.
tomschlick/framework,cybercog/framework,mul14/laravel-framework,notebowl/laravel-framework,laravel/framework,halaei/framework,mul14/laravel-framework,leo108/laravel_framework,bytestream/framework,jerguslejko/framework,stidges/framework,arturock/framework,tomschlick/framework,samlev/framework,tomschlick/framework,lucasmichot/framework,notebowl/laravel-framework,stidges/framework,cybercog/framework,JosephSilber/framework,vlakoff/framework,morrislaptop/framework,mul14/laravel-framework,tjbp/framework,crynobone/framework,laravel/framework,bytestream/framework,jerguslejko/framework,samlev/framework,mul14/laravel-framework,dwightwatson/framework,jerguslejko/framework,stevebauman/framework,lucasmichot/framework,arturock/framework,dwightwatson/framework,tjbp/framework,jerguslejko/framework,leo108/laravel_framework,hafezdivandari/framework,JosephSilber/framework,ChristopheB/framework,morrislaptop/framework,JamesForks/framework,tjbp/framework,crynobone/framework,arturock/framework,driesvints/framework,vlakoff/framework,halaei/framework,stevebauman/framework,ChristopheB/framework,JamesForks/framework,vlakoff/framework,cybercog/framework,stidges/framework,halaei/framework,hafezdivandari/framework,ChristopheB/framework,driesvints/framework,stidges/framework