Automatic merge from submit-queue. If you want to cherry-pick this change to another branch, please follow the instructions here: https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md.

promote TokenRequest and projection to beta in 1.12

```release-note
TokenRequest and TokenRequestProjection are now beta features. To enable these feature, the API server needs to be started with the following flags:
* --service-account-issuer
* --service-account-signing-key-file
* --service-account-api-audiences
```