feat: [GkeHub] add k8s_git_version field (#8586)

* feat: add `k8s_git_version` field

docs: correct comment for `k8s_version` field

The new field `k8s_git_version` captures the full Kubernetes version string for a membership.

PiperOrigin-RevId: 805935629

Source-Link: googleapis/googleapis@780e612

Source-Link: googleapis/googleapis-gen@d0c6d1d
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