Update KEP-740 for GA promotion

[HarshalNeelkamal]

• One-line PR description: Promote KEP-740 to GA/stable

• Issue link: API for external signing of Service Account tokens #740

• Other comments:
  • externalization of legacy controller in staging repo was dropped in beta promotion; Going GA without it since none of the known consumers of this feature would use the externalized controller code.
  • In production use by more then one providers i.e. GKE and EKS

/sig auth
/cc @liggitt @micahhausler
cc @ahmedtd

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @HarshalNeelkamal. Thanks for your PR.

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[liggitt]

/ok-to-test