Switch to go-katapult@next in new Provider Framework implementation

[jimeh]

When krystal/go-katapult#150 is completed, there will be a new "next" version of the go-katapult client, that is generated from Katapult's OpenAPI schema.

We should try switching the new interval/v6provider package over to this new client. This will be the first real-world use of the new client, serving as a good way to evaluate it, and make adjustments to it if needed. While also slowly moving the Terraform provider codebase forward as well.

[jimehk]

@ikadix Can this be closed now that #127 is merged?

[ikadix]

Yup happy for this to be closed