Use newest CUE (0.13.x - needed for registry support)

[MPV]

As it's needed for downloading k8s as in:

• https://github.com/myitcvscratch/yamlpodspec

[MPV]

Superseded by #83, which adopts CUE on current master.

This bumped CUE to 0.13.x for Central Registry support. #83 builds on cuelang.org/go v0.17.1 and does not need the registry at all: the Kubernetes definitions are generated from the vendored k8s.io/api with cue get go (make schema) and checked in, so there is no registry fetch at build or run time and the schema version is pinned to the k8s.io/api already in go.mod.

Closing as obsolete rather than rejected — the cue branch is untouched. #83 is one of four candidates for #26 (#81 #82 #83 #84), each measured in its description.

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