[RFE] Update Schema Definitions to use openapi v3

[MbolotSuse]

Is your feature request related to a problem? Please describe.

Currently, the steve schema definitions endpoint primarily uses the openapi/v2 enpdoint to retrieve definitions for types. This approach has several limitations:

• Fields which aren't able to be expressed in openapi's v2 syntax have incomplete defintions (see #45157). This will likely require us to fetch/process not only all openapi/v2 definitions, but also all CRDs.
• CRDs which have no definitions are returned as empty proto.Maps rather than proto.Kinds, and are missing several common fields such as apiVersion, kind, and metadata (see #45158).
• Any future removal/deprecation of openapi/v2 would necessitate rapid changes (note that this doesn't seemed to be planned at the moment).

Describe the solution you'd like Rather than using a combination of openapi/v2 and crds, steve should source the definitions from the openapi/v3 endpoint. Note that this should use client-go's built-in OpenAPIV3() functionality.

Describe alternatives you've considered N/A.

Additional context OpenAPI V3 went GA in 1.27. Prior to this point, it wasn't guaranteed that this endpoint would be available. Because of this, before this feature is implemented, the following needs to be done:

• Rancher needs to exclusively support 1.27+ versions of Kubernetes.
• We need to confirm that it is no longer possible to disable this endpoint (it appears to not be something that can be disabled, but this needs to be double-checked).

[tomleb]

We were unable to do this for the schema definition fix (https://github.com/rancher/rancher/issues/45157) because the kube-openapi library doesn't parse the references properly. So the definition schema ends up not having anything from ObjectMeta, managed fields, etc.

I've opened the following https://github.com/kubernetes/kube-openapi/issues/477 to see if we can get that addressed.