Authenticate GitHub API call in K8s version discovery

[manusa]

Summary

IsInGitHubKubernetesVersions creates an unauthenticated GitHub client, which causes
rate-limit errors (HTTP 403) when running minikube within a GitHub Action where
GITHUB_TOKEN is available.

This change reuses the existing oauth2 pattern from pkg/minikube/download/gh/gh.go
so that the client respects the GITHUB_TOKEN environment variable, falling back to
an unauthenticated client when no token is set.

Changes

• Extract GitHub client creation into a newGitHubClient package-level variable
  for testability (same pattern used in cmd/minikube/cmd/delete.go,
  cmd/minikube/cmd/start.go, etc.)
• Use GITHUB_TOKEN with oauth2 when available (same wiring as gh.go)
• Add unit tests for both the release lookup logic and the auth wiring

Follow-up

A cleaner refactor could export a shared NewClient helper from
pkg/minikube/download/gh to avoid duplicating the oauth2 wiring across packages.
Would this be a welcome change? Happy to send a follow-up PR if so.

[k8s-ci-robot]

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This pull-request has been approved by: manusa
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