require backpack as a dependency

[pxpm]

https://github.com/maurohmartinez/backpack-compare-operation/blob/5986b11b163eac57d3ba17c82302e147329a5d5f/composer.json#L7

Backpack already has supported Laravel versions and PHP versions. No need to explicitly define them. If this operation only works in backpack, it's easier if you just add backpack/crud: ^6.0 as the composer dependency 👍

[maurohmartinez]

Thanks! Done :)