branchprotector doesn't work for private repos on the "free" plan

[smg247]

Recently, GH apps have been unable to modify BP settings for private repos on the "free" plan. The tool errors like the following:

{"component":"branchprotector","error":"update red-hat-data-services/kubeflow: update master from protected=true: get current branch protection: the GitHub API request returns a 403 error: {\"message\":\"Resource not accessible by integration\",\"documentation_url\":\"https://docs.github.com/rest/branches/branch-protection#get-branch-protection\",\"status\":\"403\"}","file":"sigs.k8s.io/prow/cmd/branchprotector/protect.go:152","func":"main.main","level":"error","msg":"0","severity":"error","time":"2024-11-20T14:33:46Z"}
{"component":"branchprotector","file":"sigs.k8s.io/prow/cmd/branchprotector/protect.go:154","func":"main.main","level":"fatal","msg":"Encountered 1 errors protecting branches","severity":"fatal","time":"2024-11-20T14:33:46Z"}

The docs State the following:

Protected branches are available in public repositories with GitHub Free and GitHub Free for organizations, and in public and private repositories with GitHub Pro, GitHub Team, GitHub Enterprise Cloud, and GitHub Enterprise Server. For more information, see GitHub's products in the GitHub Help documentation.

This issue is to serve notice that others might encounter this problem, and that prow's docs should be updated to reflect this.

[smg247]

branchprotector could potentially be update to could check this itself, and just ignore the repo and emit warnings