Update Imgix dependency to "dev-main"

[kooped]

Imgix has changed their version from dev-master to dev-main commit

When using Composer 2, I'm getting the following error:
Problem 1 - plato-creative/silverstripe-imgix 4.0.3 requires imgix/imgix-php dev-master -> no matching package found. - plato-creative/silverstripe-imgix 4.0.2 requires imgix/imgix-php dev-master -> no matching package found. - plato-creative/silverstripe-imgix 4.0.1 requires imgix/imgix-php dev-master -> no matching package found. - Installation request for plato-creative/silverstripe-imgix ^4 -> satisfiable by plato-creative/silverstripe-imgix[4.0.1, 4.0.2, 4.0.3].