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

svcacct: make token authenticator fully generic

so it can be used for both new and legacy svcacct tokens. Also move the
legacy validator into legacy.go.

part of https://github.com/kubernetes/kubernetes/issues/58790

```release-note
NONE
```