[VEP 17] Graduating instancetype.kubevirt.io to v1

[Felix Matouschek]

Hi all,

the instancetype API (`instancetype.kubevirt.io`) has been served as
`v1beta1` since KubeVirt v1.0.0. It has been stable in production
across multiple releases with no breaking changes to its schema or
semantics so it is time to graduate it to `v1`.

The earlier version of this VEP included a custom sizing proposal that
would have changed instancetype application semantics to allow VM-level
overrides. After discussion this was rejected - instancetypes should
have clear semantics over how many CPUs and how much memory they
provide. Users who need defaults can use preferences, and for full
customizability there are VirtualMachineTemplates.

The proposal is now deliberately simple - no behavioral or schema
changes, just the version promotion and removal of previously
deprecated fields. The migration follows a 3-release cycle:

- v1.9.0: Introduce `v1`, deprecate `v1beta1`
- v1.10.0: Change storage to `v1`
- v1.11.0: Remove `v1beta1`

The updated VEP is here:
https://github.com/kubevirt/enhancements/pull/272

I would appreciate feedback on whether people agree that the API is
ready for v1, and if there are any concerns with the proposed migration
timeline.

Thanks,
Felix