Convert Knative Prow into a GitHub app

[chizhg]

As the number of repos in Knative grows, there'll be issues in authenticating to the GitHub APIs with the GitHub Personal Access Token, since the rate limit is always 5000 per hour. To make it more scalable and secure, it'll be nice to convert Knative Prow into a GitHub app.

After it's converted into a GitHub app, the Prow jobs for peribolos, label_sync and branchprotector should also be switched to authenticate as GitHub app, to bypass the token rate limit.

More contexts in https://docs.google.com/document/d/1q9NagH0xB2vcuK-TC_X6V_LHc0syBrNrLqiuAQ8zGWY/edit#heading=h.66y4kqbj468a

[upodroid]

kubernetes's prow instance has been running as Github App for sometime now.

References:

• Migrate Prow GitHub authentication from Personal Access Token to a GitHub App kubernetes/test-infra#10423
• Update getting started docs to suggest using an app instead of token kubernetes/test-infra#23630
• https://github.com/kubernetes/test-infra/blob/master/prow/getting_started_deploy.md
• Prow as a GitHub App design doc

[chizhg]

/assign

[chizhg]

Followed the documentations to manually apply the changes and manually did the test. Confirmed the migration has been successful.

Below are the PRs needed for the migration:

• Migrate Knative Prow to a GitHub app GoogleCloudPlatform/oss-test-infra#1553
• Remove Prow config for google/knative-gcp GoogleCloudPlatform/oss-test-infra#1554
• Move the label-sync, branchprotector, peribolos Prow jobs to authenticate as GitHub app #3198

[chizhg]

One more fix for the GitHub ops Prow jobs:

• Fix label-sync and peribolos Prow job config as we migrated Prow to GitHub app #3200

[chizhg]

The migration has been done. Marking this issue as fixed.