Migrate away from custom GitHub action approval workflow

[sbueringer]

A while ago we added this GitHub action (https://github.com/kubernetes-sigs/cluster-api/blob/main/.github/workflows/pr-gh-workflow-approve.yaml) to automatically approve GitHub actions to run when /ok-to-test is added.

Recently Prow was improved to do this automatically: kubernetes-sigs/prow#194

We should take a closer look at kubernetes-sigs/prow#612, enable the feature for CAPI and then remove our custom worfklow

We should migrate away (at least) in the following repos: (once we verified with the CAPI repo that it works as expected)

• cluster-api
• cluster-api-provider-vsphere
• controller-runtime
  • Enable trigger_github_workflows in test-infra
• controller-tools
  • Enable trigger_github_workflows in test-infra

(Just listing some repos that I'm aware of)

[sbueringer]

@sivchari Maybe something interesting for you?

(cc @fabriziopandini for awareness)

[sbueringer]

/triage accepted

[sbueringer]

/help

[k8s-ci-robot]

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[karimzakzouk]

/assign