Unpin gateway-api/conformance

We pinned https://github.com/kumahq/kuma/pull/17392 because there wasn't a released version with https://github.com/kubernetes-sigs/gateway-api/pull/4934 which makes it possible to run our GAMMA tests. Once `kubernetes-sigs/gateway-api` releases that as 1.6.1 or 1.7.0 we should change to that.